Last update: March 26, 2024

Water & Aquarium Temperature (It’s pretty darn important)

You might not be able to see it, but your fish can certainly feel it.

I’m talking about water temperature, and as you will soon learn, it’s pretty a pretty big deal.

In fact, if you don’t monitor the aquarium temperature, then you are putting the lives of your fish at risk!

See, I told you it was important.

But don’t panic just yet.

Today, I am going to teach you everything you need to know about water temperature, including why it’s so important and how you can maintain the perfect aquarium temperature.

Temperature in the wild vs in your tank

Aquarium thermometer strip on glass of aquarium to measure Aquarium Temperature

All fish have an ideal temperature range where they will thrive. Yes, I said range – in most cases, your fish do not need an exact aquarium temperature.

This makes sense, I mean, in the wild, there is no aquarium heater ensuring that the water temperature remains constant. Rain, hot summer days and cold winter nights all work together to ensure that fish in their natural environment do not experience the same temperature for too long.

In fact, freshwater fish can experience daily temperature swings of 30°F or more. Even in the reef environment, the daily temperature swings from 68-90°F (20-32°C).

So if this is the case, then why do we insist on keeping our aquariums at a constant and stable temperature?

Well, it all comes down to how fast the aquarium temperature swings.

Because oceans and rivers contain so much water, it takes a long time for the temperature to rise or drop. The change in temperature can take a day or even a season to complete.

Your aquarium, on the other hand, is not so lucky. The small amount of water can quickly heat up or cool down based on the temperature outside the tank. It is this rapid change in temperature that is the problem.

So in your home aquarium, the goal is to maintain a water temperature that is within the preferred range of your fish, invertebrates, plants or corals.

Temperature and fish health

What do you do if you feel cold?

You probably put on a sweater or turn up the thermostat. I know I would.

On the flip side, what would you do if you feel too warm?

Turn on the air conditioning or grab a refreshing drink, right?

Unfortunately, these are not options for your little fish.

You see, your fish are cold-blooded. And, it might surprise you to learn that this does not mean that their blood is cold.

Instead, it means that your fish cannot regulate their internal body temperature – as the water temperature changes, so does their body temperature.

Water that is too warm or too cool becomes uncomfortable.

In the wild, a cold fish would simply swim to warmer water. Similarly, a warm fish would hunt out a nice cold spot.

But the fish in your aquarium? They don’t have this luxury and are forced to endure the aquarium temperature… There is no escape, even if they wanted to!

Unfortunately, this temperature can have a negative impact on the health of your fish. In particular, the water temperature has a direct effect on your fish’s metabolism.

If your tank temperature is too cold, then your fish’s metabolism will slow down, which leads to your fish becoming sluggish and sleepy.

On the flipside, water that is too warm causes your fish’s metabolism to speed up. Your fish will become more lively or even hyperactive.

While that might not sound too bad, these rapid fluctuations in temperature have been proven to cause stress. And, you don’t want that! Stress is arguably the number one reason why aquarium fish die

You see, the stress caused by fluctuating temperature weakens your fish’s immune system. Even fish that are briefly exposed to temperature swings are less resistant to disease and parasites like Ich – which feed on the fins, skin and gills of your fish.

Sick goldfish in fish bowl with thermometer and ice pack - importance of Aquarium Temperature

Normally, your fish’s immune system would easily fight these off – they are no match for healthy fish. But when temperature-stressed, your fish are an easy target for these diseases and parasites, which can soon kill them.

The life of your fish depends on your ability to maintain the correct temperature.

Don’t have fish? You’ll still need to keep an eye on the temperature. Invertebrates, corals, plants and algae have all been observed to be negatively influenced by temperatures outside of their preferred range.

Even the beneficial bacteria in your aquarium are affected by water temperature. The preferred range is 65-85°F (18-29°C). At 95°F (35°C), bacteria die, which can lead to ammonia spikes. Lower temperatures cause the bacteria to become less active and can even make your aquarium take longer to cycle.

As you see, it’s pretty important that you keep your aquarium at the correct temperature…

How to position the heater correctly?

One must ensure that the temperature of your fish tank water is consistent with the fish species you intend to keep, as these creatures are naturally accustomed to warm/cold water.

If you are setting up an aquarium, the temperature should be similar to your fish’s natural environment. You should place the heater behind the aquarium or tank.

Or, at the least, make sure it is near a primary source of water flow. It should not be in direct contact with other objects that can block its heat, such as rocks, plants, or substrates. Place it so that when you look at your fish tank from the front, it is on the right or left side where there is a lot of movement in the aquarium water to keep it circulating correctly.

It’s also essential to place any thermometer you’re using next to your fish tank because heaters work less efficiently if they’re too close or too far away from them.

When setting up your aquarium, you should ensure the heater does not contact the walls or substrate. If your heater has a glass casing for protecting its internals, the significant difference in temperature across the layer of material could crack your enclosure and potentially harm your fish.

Providing your tank with consistent, sufficient heat is one of the many ways to ensure that all of your fish are comfortable and able to thrive.

If you have an aquarium heater, then the best place for it is in a location where the water passing over the device makes constant contact so that the conduction and convection process can effectively transfer warmth evenly throughout your entire tank.

Another worthwhile location is near an intake or outlet tube because that way, hot spots will not form on or around your heater, which could lead to sudden power-off. It can simply create an uncomfortable living environment as these areas would be much warmer than they should be.

What happens if your aquarium temperature is too high?

Fire in aquarium with fish because aquarium temperature is too hot

Warm water causes the biggest issues, typically once temperatures exceed 90°F (32°C).

Your fish breathe by drawing oxygen out of the water. And under normal conditions, your aquarium will have more than enough oxygen to go around.

Warm water causes your fish to become more active and require more oxygen. Just one problem – warm water actually holds less oxygen than cooler water. In severe cases, there won’t be enough oxygen to go around, and your fish can suffocate.

To add to the disaster, the beneficial bacteria in your aquarium need oxygen to break down ammonia. Without enough oxygen to go around, the ammonia in your tank begins to build up, displacing oxygen and burning the gills of your fish.

As you see, warm water can kick-start a number of deadly problems in your aquarium.

What to do if the temperature gets too high?

There are several ways to keep an aquarium cool. Aquarists will have to figure out what works best for them by considering the circumstances at hand and the species of fish they have in their tanks.

What works for one won’t necessarily work for another, so it is essential to consider the situation and make a plan accordingly.

Air conditioning

Out of all the elements used to keep an aquarium at appropriate temperatures effectively, there is one that you should be sure to have: a cool exterior environment.

By maintaining a cool temperature around the aquarium, it will, in most cases, ensure that the temperature within the tank doesn’t rise beyond safe range.

Frozen water bottles

Frozen water bottles make the perfect temporary ice cubes for an aquarium. It is a cheap and easy way. Many people use this method. The frozen water bottle method works differently from regular ice cubes, which tend to melt fast and cause havoc in your fish tank.

You may have heard of another aquarium cooling system involving small glass tubes with water inside them packed in a container with the fish tank. It gives off the same effect as the frozen water bottles but is comparatively more expensive to buy due to its components being replaced time after time.

Run a fan directly on the surface

This is a quick and easy solution to the problem of stagnant water at the tank’s surface. The downside is that more water will need to be added to replace what has evaporated since it increases evaporation rates.

Reduce natural and artificial lights

Install blinds in your aquarium room to block out light when temperatures rise as too much artificial lighting can seriously damage tank inhabitants, too. As artificial lighting has been known to introduce algae blooms that can quickly spread, we suggest turning off lights in the aquarium if not needed while working in the room.

Install a good filtration system

Use proper water filtration equipment by installing water-filtration systems and new filters each time they become clogged. It will keep water quality healthy and ensure that the fishes inside are flourishing without risk of harm coming to them.

What happens if your aquarium temperature is too low?

Snowflakes in aquarium with fish because temperature is too cold

For tropical fish, low temperatures can also cause a problem. At lower temperatures, these fish can become slow and sluggish, and many will soon die.

It’s similar to how you can survive outside without clothes or bedding. However, the experience would be uncomfortable, and you would soon catch a cold or the flu.

What to do if the temperature gets too low?

As fish are cold-blooded, they have a high sensitivity to changes in temperature. You must maintain aquarium temperature with dependable equipment for your aquarium heaters.

Invest in good water heating equipment

A good heater will have a built-in thermostat that can automatically turn the heater on if the temperature changes. Some heaters have a single temperature setting, but more advanced models use a timer or sensor to regulate the tank’s temperature. Many people position their heater in the middle of the tank to help keep tank temperatures stable, and there are particular types of heaters that work best for specifically sized tanks.

Get a thermometer

You should also invest in a thermometer that will help you monitor the tank’s temperature to keep track of it and take necessary steps as required. A sticker thermometer will help you to keep track of the temperature, so you should be sure that they are checked every time you view your tank at home. Stick-on versions are relatively inexpensive and can be read without distortion by looking at them from far, and immersion versions are also good but more expensive than stickers.

Additional ways to cool your aquarium water

  • Position your aquarium against a wall that reduces outside air drafts in which the warm air rises.
  • One way you can make your fish tank warmer is to increase the temperature in the room where the aquarium is located by setting it to 78°F (25.5°C).
  • Keep the aquarium in a warmer area of your home or closer to a heater.
  • Occasionally, putting warm water into the tank can help you maintain the temperature without using a heater.
  • After getting the temperature of your tank at a comfortable temperature for the fish you’re planning on housing, consider insulating your glass walls.
Optimal temperature for betta fish

Here’s where things get a bit tricky. Each fish has its own preferred temperature range and tolerance to temperature swings.

Generally speaking, when it comes to temperature preference, fish will fall into one of three categories:

  • Cold-water fish: Below 68°F (20°C)[1]
  • Tropical fish: 75-80°F (24-27°C)[2]
  • Temperate fish: A wider range of temperatures, often overlapping the above categories.[3]

The temperate fish category is a recent creation to cover fish that don’t fall into the cold-water or tropical categories, like goldfish. Even so, temperate fish still have a preferred temperature range where they will thrive.

Typically, fish that come from large bodies of water, such as oceans or large lakes, are generally less tolerant of temperature swings. This is because these large ecosystems have stable temperatures, so the fish have not adapted to fluctuations in temperature.

It’s up to you to research any fish that you plan on adding to your aquarium to figure out its preferred temperature range. Usually, the place you are buying your fish will provide that information – all you need to do is ask.

Oh, and it goes without saying that the temperature of your tank will determine the fish that you put inside it.

But be careful!

Just because two fish can live at the same temperature, doesn’t mean they should be added to the same tank.

I see this mistake made by beginners all the time. Take the following two fish for example:

  • Fish 1: 64-72°F (18-22°C)
  • Fish 2: 72-82°F (22-28°C)

Both fish can survive at 72°F (22°C). As long as you keep the temperature at precisely that, the fish can live together, right?

Unfortunately, this isn’t the case. In this instance, both fish would be living at the edge of the temperature range that they find most comfortable, which can lead to both fish becoming stressed and having shorter lives.

In the wild, fish don’t experience the edges of their temperature range for long. Keeping your tank on the edges of this temperature is like creating a permanent summer or permanent winter for your fish.

It goes without saying that some fish are more tolerant to temperature changes than others. And those of you with experience will have a better understanding of exactly what temperatures your fish can tolerate.

But for any beginner starting out, stick to the middle of the temperature range – it leaves less room for error.

Remember: Just because your fish is surviving, doesn’t mean it is thriving… Fish have recommended temperature ranges for a reason. Just because your fish is alive at a temperature outside of this range doesn’t mean it is happy or healthy.

Suitable temperature for different fishes

Temperature is just one factor we need to consider when caring for our aquatic pets; others include water quality and a healthy diet. Too high or too low, and your tropical fish could fall ill.

Goldfish prefer constant temperatures between 68° and 76° F, while cold-water species thrive at around 65 ° F or below. Temperate fish live in overlapping habitats with tropical and cold-water species, and thus, they can be treated the same temperature-wise.

Freshwater aquariums are designed to be used in areas not exposed to direct sunlight and with temperatures between 72ºF – 82ºF, depending on the type of freshwater fish you intend to use.

Some fish prefer cooler water (67°-70°F) while some enjoy warmer waters (80°-86°F). It is advised to find out what will best match your preference and the fish you have chosen for your tank.

Finding the right temperature for a saltwater tank isn’t very straightforward either. Saltwater fish come from oceans (or bodies of water) worldwide, and their environments vary slightly from one to the next.

You should base the water temperature of your tank on what types of fish you put in it. Generally, 79 degrees F (26°C) is an excellent medium but, again, check with your local marine life store or internet to see which range they prefer.

Be aware of the effects increased temperature can have in your fish- and coral tanks! Most reef tanks are found naturally at a temperature between 82 to 84 degrees F (29°C), and it’s standard advice to keep a reef tank at 78 – 80°F (25°C).

However, as our ambitions are to maintain, breed, and raise fish, most species will find the environment more than comfortable at about 77 – 78 (25.2 – 25.4) °F!

Following are water parameters for different aquarium types:

Aquarium type Suitable temprature Suitable pH levels
Tropical fish tank 75-80°F (24-27°C)  6.5-7.5
Shrimp tank 75-80°F (24-27°C) 6.5-7.5
Planted tank & Discus 75-80°F (24-27°C) 6.0-7.5
African cichlid tank 75-80°F (24-27°C) 7.4-8.6
Central American cichlid 75-80°F (24-27°C) 7.0-8.0
South American cichlid 75-80°F (24-27°C) 6.0-7.5
Brackish tank 75-80°F (24-27°C) 7.5-8.4
Saltwater fish & live rock (FOWLR) 75-80°F (24-27°C) 8.1-8.4
Reef tank 75-80°F (24-27°C) 8.1-8.4
Pond 33-86°F (0-30°C) 6.5-7.5
Axolotl tank 60-64°F (16-18°C) 6.5-8.0
Turtle tank 72-79°F (22-26°C) 6.0-8.0

Breeding Temperature

what is the best temperature for breeding

Most fish require a specific temperature to breed. Sometimes that temperature is higher than the standard range, but breeding is triggered by a drop in water temperatures in many cases.

During fish breeding, it’s essential to know the water temperature requirements of the species being spawned. This way, you can adjust the temperature in another tank specifically for these fish’s needs. In that way, they don’t feel like they are being left out or surrounded by too many other fish.

The correct water temperature and pH should be considered while breeding fish. Certain species of fish can breed even in acidic water, but others can’t breed in water not as acidic as they require. You can adjust the ideal water temperature and pH according to the species of fish one wishes to breed.

For example, the correct temperature for breeding bettas is 28°C (82.4°F) for the first week and lower afterward. The pH should be at 6.5 to 7.5. Fish breeders usually get this temperature from a chiller or from a room where the temperature is constant.

The most vital thing is to keep the temperature stable; otherwise, the eggs will catch fungus. A buffer is needed to get the temperature right, preventing pH shock and stabilizing the pH. When adding the buffer, it should be done gradually to stabilize the pH, making the pH less acidic and more healthy for the eggs.

The best breeding temperature for fish changes depending on the fish variety. There is no single, standard temperature. For example, in general, tropical fish can breed at a temperature of 22 C (71.6 F). But some species do better at 25 C (77 F), while others have a lower limit at 18 C (64 F).

There is also a wide range of water temperatures that fish species can tolerate. For example, you can keep goldfish in water at a temperature of 18-30 C (64-86 F) and cichlids in water at 25-28 C (77-82 F). The average home aquarium temperature is 72F, but fish like Discus and Angels prefer higher temperatures.

Fish species do not have a single reproductive cycle each year. Some tropical fish can breed at any time of the year, so the temperature is not a big issue here. On the other hand, if the temperature is too low, they can’t breed.

How do you maintain the ideal temperature in your aquarium?

Aquarium fish next to thermometer with perfect temperature range

Now that you know why the correct temperature is so important, it’s time to maintain it.

If you know what the temperature is inside your aquarium, then half the battle is won. Unfortunately, warm water looks the same as cold water.

That’s why you need an accurate aquarium thermometer. It really takes the guesswork out of monitoring your aquarium and is an essential purchase for every aquarium!

With your thermometer in hand, it’s time to take a closer look at how you can maintain the perfect temperature for your fish.

Getting the basics right…

It all starts with where your fish tank is located.

Drafts from open windows or AC vents quickly cause the temperature in your aquarium to plummet.

Similarly, the midday sun that is shining through your windows, hitting your aquarium, can cause the water temperature to soar.

This is why it’s so important to choose the correct location for your fish tank when setting it up!

Heating your aquarium

Aquarium too cold? It’s time to turn up the heat!

Heating your tank is darn simple. All you need is a good aquarium heater. This handy little device sits underwater and will continue to warm your water until your desired temperature is reached.

Cooling your aquarium

If you live in a warmer area (looking at you, Arizona!) then you might have the opposite problem. The hot weather makes your aquarium water too warm.

Leaving your AC cranked is overkill, not to mention expensive! The cheapest solution is to use a fan. While any old fan will do, there are fan systems specifically designed to keep your aquarium at the correct temperature.

Those of you with larger tanks might find that a fan isn’t enough to lower the temperature. In this case, you have the option of using an aquarium chiller – essentially a canister filter that cools the water as it flows through.

Maintaining the temperature automatically

Wouldn’t it be great if you could set up your heating and cooling equipment to remain switched off and only turn on when needed?

An aquarium heater controller is the exact product you are looking for!

Simply plug in your heater and fan, set the desired temperature and walk away. If the water temperature drops below this setting, the heater will automatically turn on to warm your water back up.

Similarly, if the water temperature gets too hot, the controller will turn your heater off and turn your fan on, to cool the water back down.

All without any effort on your part. Pretty cool, huh?

Best Temperature for Your Aquarium

One key to fish-keeping is temperature. Tropical fish – the kind you can buy at the store – are most comfortable between 75 to 80 degrees Fahrenheit (24 to 27 degrees Celsius).

Cool water fish, like goldfish and koi, need water that’s much cooler than that – between 60 and 75 degrees F (15 and 24 C) or even colder. Remember: Goldfish don’t do well in temperatures above 80 degrees F. So if you have a fancy tropical fish tank, do not put a cold-water fish like a goldfish into it!

The temperature for your fish depends on the species of fish included in your aquarium. Figure out what kind of fish you are interested in keeping before setting up a tank and compare their temperature requirements with those of the water and heater you plan to use.

Only go with a reliable thermometer to regularly check the water temperature to ensure that any rapid or significant changes avoid sudden alterations in the climate inside the tank.


A stable water temperature, within a few degrees of the recommended range, is important for keeping everything in your aquarium happy and healthy.

With the help of heaters and chillers, keeping your aquarium at the proper temperature has never been easier!

Do you have any temperature tips? Let me know in the comments below!

Ian Sterling

Ian Sterling, founder of, began his aquarium journey over 30 years ago, driven by a deep fascination for fish and their diverse personalities. His website,, is dedicated to making fishkeeping accessible and enjoyable, offering beginner-friendly guidance, expert insights, and a community for aquarists to connect and share experiences.

Comments (58)

Very helpful reviews! I had two aquaria with lots of fishes in it. But they die frequently. I couldn’t find the reason behind it. Later on, I have found that my fishes die due to hot temperature in water. your article helped me a lot to find the solution for my aquaria. Thank you for sharing your reviews.

Thanks for sharing knowledge . I have a 80 gallon tank with discus and plants . Could you please suggest what temperature should I maintain . Beside that where should I put my thermometer to check temp . I am using digital controller

Thanks ahead .

Hi Amit,

It would all depend on the species of discus. Practical Fishkeeping has a great guide on this:

Generally speaking, between 82˚F and 86˚F, although their are some discus that prefer it a little warmer. You’ll have to research the specific discus in your tank. just remember – it’s more important to keep temperature constant than exact.

While changing water in aquarium unfortunately cold water was given erroneously more som some fishes were go dying. Though I immediately added hot water to maintain 28 degree as required. Fishes look survived.
Any thing more to do?

Hi Madhurendra,

Good save. Depending on the water source, your hot water heater may have trace amounts of copper – this comes the heating element. It’s likely such a small amount that you won’t need to worry about it unless you have particularly sensitive fish, plants or corals.

Hi, I have 6 Tiger Barbs, 2 Nerite Snails, and 1 Ghost Shrimp (which surprisingly gets along with the Tiger Barbs, even though everyone said they wouldn’t ) in my 20 gallon tank. I keep the temperature of my tank at 75 degrees, although at times the water temperature drops a few degrees during water changes because I have to unplug the heater and would rather use colder water than too hot water when adding new water. (I heard that it was actually good for the fish to be exposed to slightly cooler water for a little bit, it’s apparently refreshing. Is this correct?) Also, is 75 degrees an okay constant temp. or should I make it a bit warmer? (the next level up on my heater is 78…)

Thank you so much!

Hi Amelia,

I’m glad to hear everyone in your tank is getting along.

I’m not sure where you heard “cold water is refreshing” but I can assure you fish don’t find sudden temperature drops refreshing. More like stressful. Normally you would try to match the temperature of your tank when performing a water change.

As for temperature, read up on the preferred temperatures of your inhabitants and pick a happy middle ground. If that’s 75 or 78, it doesn’t matter as long as it’s constant.

Okay, thank you. I’m a bit confused, though. If it stresses them out when the water drops a few degrees when I do a water change, won’t it stress them out too if I higher the temperature to 78 degrees in the tank?

The heater won’t instantly change the water temperature. It needs time to heat the water around it. The change can take a few hours. This is normal, like what happens in their natural habitat across a day as the sun hits the water and sets at night. When you do your water change, the temperature change is instant. It’s this rapid change that stresses fish.

Hi Ian

I would love some advice. We have 9 small koi (approx. 4 cm each) in a 60 litre tank. We were advised to keep our water at 26 degrees (as per all other tropical fish). The more I read, the more I realise the water should be a lot cooler, even simply room temperature. How to I transition to a room temperature tank – do I lower the water heater by 1 degree each day or so?


Hi Robyn,

Whether raising or lowering the temp, slow is best. Your transition of 1 degree per day would be fine. If you wanted to, you could do it quicker, say 1 degree every 3 hours or so.

I’d be more concerned with how many koi you actually have in your tank than the temperature. nine 1.5 inch fish in a 15 gallon tank is dangerously overstocked.

It’s generally recommended you only stock 1 inch of koi per 10 gallons of water while they are growing – so if a single koi is 5 inches long, you would need a 50 gallon tank for that one fish. There is a reason why Koi are generally kept in ponds. However, this is outside the scope of this article, so I suggest doing some more research here. Good luck!

Ian, thank you for your accurate advice. You are absolutely 100% correct about the koi being overstocked. It boils down to research before hand, which we did not do. Also, it is amazing how many pet shops will sell you fish without the correct advice. I have since managed to re-home the koi.

That’s fantastic to hear you were able to re-home your Koi. Don’t blame yourself on research, in the current year you would expect to walk into a petstore and receive accurate advice on how to care for the pet you are purchasing – I don’t think that’s unreasonable. Advice that is common knowledge among fishkeepers isn’t repeated at the storefront. Bigbox pet chains are the worst at this. It’s a big reason why the “you can add fish to your tank on day one” myth won’t go away. Independent fish stores are generally much better. If you are going to give this hobby a second chance, I recommend using your local one, the owners are typically much more knowledgeable as they raise their own fish too. Wishing you all the best for the future!

Mr. Ian… I have a 20 gallons aquarium. I have 3 gold fish. My water tank gets so cold in winter time… should I buy a heater to warm the water.. and If i buy it.. what temperature should it be?

Hi Edgar,

I’d be more concerned with the size of the aquarium than the temperature. If your temperature is rapidly fluctuating then a heater is advisable.

I recently purchased a water changing system. I live in Florida, since it is currently winter it is easy to warm up cold tap water to temp. In the summer, the tap water can be in the mid to upper 80s (f). What is the temperature differential tolerance for adding warmer water to a tropical community tank? Will 60 gallons have enough have existing water (25% water change) to be stable? What about 10 gallons? A 29 gallon goldfish tank?

Hi August,

that’s more a maths related question than one I could tell you off the top of my head. I have never had to deal with that situation, so cannot even give you a ballpark figure from personal experience. When I’ve been in Florida, the cold water has never reached that high. Even on a hot day. Is your situation different? The only thing I could suggest is use an accurate thermometer and slowly replace the water, monitoring it closely. Eventually you’ll get a feel for the tolerance.

Hi Mark,

Great question! For larger tanks like yours, I prefer two as a safety precaution. If one blows, the other will stop the temperature falling as quick. Similarly, if one gets stuck “on” it won’t cook your fish because it’s not powerful enough.

I am considering turning my in ground pool (40x 17)into a pond. My Wife and I still want to swim in it but we like the water to be at 85 degrees. What LARGE tropical fish can thrive at 85. We will run the heater year round. We live in Los Angeles.

Hi Mark,

85˚F is on the extreme of what most fish would experience. I wouldn’t recommend it as a swimming pool, it’s the perfect temperature for bacteria to flourish. Ordinarily chlorine and other chemicals would keep this safe for humans, like in the case of a hot tub, but if you were to add this, you would kill your fish.

I have a 1000-gallon tank that I am using a moving bed filter to treat the water. I have the tank and filter as air tight as I can get them but with the moving bed filter adding a lot of forced air in to the filter & tank I am worried about the humidity that could possibly be getting out in to my room. I heat the water to 75 degrees and my room is any ware from 62 to 70 degrees. I am thinking about installing an air vent from the tank and filter to the outside. Right now, I have no problems, but It is winter and air inside the house is dry from my home heater running a lot. What are your thoughts, and do you have any suggestions?

Hi Matt,

Unfortunately, the answer here is: “it depends” I’d personally take the wait and see approach, before spending time and effort installing an airvent with a fan if it’s not needed. I’d suggest buying a humidity monitor, they can be picked up for cheap and monitoring the situation, reacting accordingly.

Wow! That is some dedication!

I have a small 300-liter fridge and might be away for these summers with someone feeding them twice and cleaning the aquarium fortnightly.
However, my house gets HOT. Hot to 45*C+. And, I do not have airconditioning.
If I leave the fish alone in this house, they shall be roasted!


I just up graded to a 75 gallon tank and the weird thing is happening, the temperature keeps going up and I have the heater off. I just don’t know what to do.helps!!

Hi Mary,

Is the tank sitting in direct sunlight? Is the is there an alternate heat source? What happens when you remove the heater completely?

With all those boxes checked, a 75 gallon tank shouldn’t heat past room temperature.

Great article!
I was wondering, I have 3 wcmm and lately (even though it’s freezing at the moment) their tank temperature has been way too high! My thermometer reading is quite high and I can even feel putting my hands against the glass of the tank that it’s much to warm! I tried to float ice cubes, but it didn’t make much difference.
I feel like there’s something wrong as it’s the middle of winter over here! I’m really beginning to worry! Help!

And I don’t want to take off then lid as I have heard that they are known to jump when spooked. Any ideas on what I could do?

Hi Sea Monkey,

I assume you are talking about White Cloud Mountain Minnows? 61F sounds like the temperature is fine?

Yes. And whoops, I meant 71F, not 61F. Double checked the temperature and realised that I’m pretty terrible at reading thermometers! Was making temperature look way higher. It’s actually only 52F.

Thanks anyways!


I have recently shifted to my new house and water temperature of my aquarium has gone high to 31 digress. I suppose you will his is due to the Kitchen which is behind the aquarium wall, heat generated in kitchen is passing by the walls.

What can I do to get this sorted.


Hi Sumit,

If it’s only that location, my suggestion would be to either move the fish tank or try place some insulating material between the tank and your wall. Alternatively, chillers are sold that can keep fish tanks cooler, but they can be a pain to run.

Hey Ian, hope you are doing well….so I just brought my red and white koi swordtails from the outdoor pond this evening….man these fish have packed on some serious size. In April they were like 2″ and now they are like 3″+ and very thick and full of babies. My question to you is how many of these should I put in my 125 gallon tank so that I don’t exceed bio-load. I am running 2 huge sponge filter and a aquaclear 110 in this tank. There is no substrate at the bottom. The temperature will be somewhere between 68-70 in the basement for the Fall and Winter months…..any advise on beneficial bacteria being able to support the bio load that that temperature range. Thanks man.

Hi Renapond,

To put it simply, koi are best suited for ponds. A 2 inch koi needs around 60 gallons. They grow quick, so it won’t be long before they outgrow your tank here. Unfortunately, your koi are likely already be too large for this tank.

I have a 55g freshwater community tank (Corys, rasbora, cherry barbs, neon tetra, Rainbowfish) with 2 thermometers. Both were supposed to be set to 74 degrees but I noticed yesterday that one of them had been changed to 91 – the dog chased the kitten back there and someone must have knocked the dial – 91 is the max temp on the dial and is only one notch over from the 72/74 setting.

The actual temp in the thermostat probably wasn’t 91 since the thermometer on the left side of the tank is still at 74, but yikes, definitely it’s too high. I don’t want to cause shock by changing the 91 directly to 74. I assume I should dial it back gradually – but how gradually?

And yes, I’ve already made some changes to make sure it doesn’t get knocked/played with again.

Hi Allyson,

These things happen. I assume you are talking about heaters with a built in thermometer? If so, dialing it back by a degree or two every couple of hours should be slow enough to avoid shock

It’s over 100 degrees Fahrenheit here in LA, I just checked my thermometer and it’s registering at 91 degrees. I have them in a dark room, no direct sunlight, the heater has been unplugged for now, and it’s still reading at 91. What do I do?

Hi Phoenix Q,

That’s quite a heat wave. If you google “how to cool aquarium” you should find a variety of DIY ways to cool your tank.

Hi Ian, I have just taken on 7 danios (quite small ones) and bought a 22L tank following advice from fish shop ( I wish i went bigger), however, the fish came from an air filtered environment with no heater. The tank has a heater and I put this on to 21degrees, I notice quite a bit of condensation at top. Should I switch the heater off again? I’m worried about oxygen saturation. Fish are in centre of house, not near heat or chill sources. Thanks

Hi Claire,

Is that condensation on the lid? That’s normal. I’d leave the heater on since it will top temperature swings.

I also had to admit that the tank was getting way too many hours of artificial light each day, because I often forgot to turn the tank lights off, or wasn t home to do it. As a result the lights sometimes stayed on for a dozen or more hours, something that wasn t necessary in a fish-only system. (If your tank contains plants or coral, you obviously have to take their needs into account when adjusting the lighting, but in such tanks algae is less likely to be a problem because the other

Hey Ian, I was wondering. I have cosmic goldfish and they have been pretty much swimming really fast back and forth against the sides of the tank. I keep the water regulated around 69-71 degrees as suggested by the people at petco. I don’t know if my fish are stressed or not. Any suggestions?

Hi Brody,

If your fish are “flashing” it could be a disease or water quality, did you cycle your tank when you set it up?. To rule out water quality, you will need to grab an aquarium test kit (I recommend the API master test kit) – it’s almost impossible to keep fish without one. Check the results and this should help you narrow down the problem.

Hi Brody,

It could be a parasite disease or stress. It’s honestly hard to say. I still recommend checking your water parameters with an aquarium test kit, it’s possible this fish is extra sensitive. You should buy one anyway and should be checking water quality at each water change at least.

Hi Ian, thank you for your article. I keep two cold water fish and I noticed that one is having problems with swimbladder, especially in the winter months. It is not bloated and stays upside-down at the bottom of the tank.
Our house is quite cold, room temp around 14 C and it doesnt get too hot during summer, therefore we never needed a water heater… But now I can only presume that maybe water temp would be too cold for the fish..? Is there anything you could recommend?
Would appreciate your help, thank you.

Hi Jess,

It’s really hard to diagnose a fish online, the cause could be from many different things, including water quality (your aquarium test kit should confirm this) If 14C is in your fish’s comfort zone, then it likely isn’t the temperature that is the issue. If you google your fish + temperature, you should find the recommended temperature range.

Dear Ian, thank you so much for your reply. Your blog is the most informative and helpful of all the info available online. And thank you for taking the time to do it…
I am on the path of doing an in-tank fish cycle, thanks to your article… All those years I never knew of the importance of it… My poor fishies! Must save them if its not too late…
Thank you again for everything!

Hi Jess,

Oh wow, that was a very good discovery that your tank wasn’t cycled. I’m glad you stumbled across that article – cycling is hugely important, it’s almost impossible to keep healthy fish in an uncycled tank. If you need any help with the process, drop me a comment in the cycling article and I’ll do my best to point you in the right direction.

Hi Ian, sorry to jump in on someone elses thread but i couldnt figure out how to start a new comment…was so glad to find your page being new to owning a betta fish, it is so informative…was wondering if you could help me…

I will start by giving a bit of background, i have kept platy fish for a number of years but only recently became the owner of a betta fish. My betta fish is male and kept in a separate tank. When i first had him i was given some poor advice by my local aquatic shop, they told me i only needed a 5 litre tank with no filter or heater…i wasn’t totally comfortable with this & so opted for a 10 litre fish bowl with filter. My betta seemed to settle in really well, very active & eating well. Then i noticed he was very lethargic & resting on the bottom of the tank a lot…this is when i started to do some more research. At first i thought it could be swim bladder from constipation so i starved him for one day then gave him some mashed up cooked peas. This didnt make much of a difference. I do regular water changes & my water parameters have always been good but i noticed the nitrite was slightly up, so i carried out water changes over 2-3 days. I managed to get the water levels back within the right range but he was still resting a lot at the bottom of the tank. I tested the amonia too & this was 0. I then thought that maybe he was lethargic because he was cold so i invested in a heater & gradually raised the water temp…again this seemed to make little difference. I then realised after more research that his 10 litre fish bowl was much too small & i was starting to wonder if he was depressed & bored…so i invested in a new 23 litre aquarium with a better filter. Since moving my fish in to the new aquarium he seems much happier & much more active, hardly resting on the bottom at all. I just want to add during all of this time his appetite has remained the same. I feed him 5 pellets once a day, brine shrimp twice a week & mashed peas once a week & give him no food one day a week. So the reason for my email is that it is only since i moved him in to his new tank that i have noticed that his fins look more ragged than usual…it doesn’t appear to be fin rot to me as i cant see any discolouration around the edges of his fins & he doesn’t have all the symptoms of fin rot so i was just wondering if you could shed any light on this…my fish has also changed colour dramatically since ive had him but im assuming this just happens with maturity? I would really appreciate your opinion and advice on this…i was also thinking about adding aquarium salt to his tank too…do you think this would help? Im happy to attach before and after photos of my fish if that would help too? Thanks in advance.

Hi Chantal,

Your first port of call would be to test the water in your aquarium. It’s almost impossible to give advice without this as a starting point.

Very helpful reviews! I had two aquaria with lots of fishes in it. But they die frequently. I couldn’t find the reason behind it. Later on, I have found that my fishes die due to hot temperature in water. your article helped me a lot to find the solution for my aquaria. Thank you for sharing your reviews.

Hi Boppi,

I’m sorry to hear about your unfortunate run with keeping fish. I hope it all goes smoother with this knowledge in hand!

When is the right time to put in the water heater when first setting up the fish tank without purchasing the fish for the next couple of days?

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