Last update: November 12, 2022

Aquarium water turn green? Here’s how to fix it (In just days!)

Uh oh… You don’t know what happened, but now your aquarium looks green?

Chances are your tank is suffering from green aquarium water. And, it ain’t pretty.

Some people say it looks like pea soup, but to me, it looks more like Mountain Dew. In any case, it’s not something you want in your aquarium, right?

Today, I am going to teach you everything you need to know about green aquarium water including what causes it and, perhaps most importantly, how to get rid of it for good.

AA Aquarium Green Killing Machine UV Sterlizer

AA Aquarium Green Killing Machine

  • Patented sealed UV bulb
  • No tools required
  • 3-year warranty
Water polishing pad for filtering green algae out of water

Polishing Filter Pad 100 Micron

  • It traps fine particles
  • Pad is made with polyester clothd
  • Cut to fit
API Algae fix to control and remove unwanted algae growth

API ALGAEFIX Algae Control

  • Control algae growth
  • Product tested and safe for salt or fresh water setups
  • 100% dedicated to performance and clean environment

What is green aquarium water?

You see that green cloud in your aquarium that keeps getting thicker and thicker?

Well, it’s actually made up of millions of tiny pieces of algae called phytoplankton.

Phytoplankton are small. Really small. In fact, you cannot see them with your naked eye. To view individual phytoplankton, you need a microscope…

Green aquarium water under microscope

It’s when these phytoplankton get together in a large group that you first notice them in your aquarium.

When low in number, these phytoplankton will turn your aquarium a slight hazy-green color.

The beginning of hazy green aquarium water

But as they grow in number, they will turn the water into a cloudy green mess….

Aquarium water beginning to turn a murky shade of green

If you don’t do anything about it, then the algae bloom can get so bad that it blocks out everything in your aquarium…

Aquarium water turning even greener

Depending on the lighting and what you have inside your tank, your water may have a yellowish-green color…

Thick cloudy green-yellow aquarium water

Rest assured, it’s the same stuff – green aquarium water.

As you see, you want to get on top of this type of algae before it takes over your aquarium. I’ll show you how to do exactly that later in this guide.

But first, I want to make sure that what you are dealing with is actually green aquarium water…

Green aquarium water is sometimes confused with a different type of algae:

Green dust algae.

And, I can see why. If green dust algae is left to grow, then the two look nearly identical…

Green aquarium water compared to green dust algae

On the left, you have green aquarium water, and on the right, you have a tank covered in green dust algae. Not much difference, right?

Telling the two apart is actually very simple. You see, green dust algae is a clinger. It will happily cover your aquarium glass, plants, substrate and anything else in your aquarium.


It doesn’t like to float through your water. Well, not unless it is disturbed. And because of this, the water in your aquarium isn’t actually green. It just looks green from the light reflecting off the green dust algae that is covering every corner of your aquarium.

Below you see what happens when you clean green dust algae off a single panel of glass.

Green dust algae coating the glass substrate, wood and plants in aquarium

The aquarium water looks clear again because the water itself doesn’t contain any algae, just the surfaces.

The algae in green aquarium water, prefers to float through your water, without settling on any surfaces.

Because of this, the two can be identified by taking a sample of water from your aquarium. If it’s clear, you likely have green dust algae. If it’s a cloudy green color, then it’s probably the algae commonly referred to as green aquarium water.

Think you might have green dust algae instead? Don’t worry! I also have a detailed guide on getting rid of green dust algae.

Note: Take care when identifying the algae in your aquarium. Algae typically appears when there is an imbalance of nutrients, and it is possible to have more than one type in your tank at a time.

Is green aquarium water dangerous to your fish and plants?

Otto on glass of aquarium with green water from algae bloom

Generally speaking, no. Green aquarium water is not considered dangerous to fish. In fact, many fish happily live in green-colored water in their natural environment.

The same goes for plants. While the algae that makes up green aquarium water feeds on many of the same nutrients that your plants do, it shouldn’t cause any direct harm.

What causes green aquarium water?

Fish and snails in fish tank with green-colored water

The general consensus among the aquarium community is that there are three main causes of green water algae. Staying on top of these causes will go a long way toward preventing green aquarium water.

1. Too much light

Phtyoplankton is a plant. Plants love light. See where I am going with this…

Too much light could be the very reason your aquarium water keeps turning green.

It always surprises me just how many people keep their aquarium lights on 24/7 when 8 to 12 hours of light a day is more than enough for most aquariums.

Another common mistake is using a light that is too strong for your aquarium. Seriously, I have seen grow-lights installed on tanks with fake plants!

So, for many of you, reducing the amount of light your aquarium receives will be all that is needed to stop the green water from appearing in your aquarium.

Rather than constantly turning your light on and off (and forgetting!), I recommend buying a good aquarium timer for your tank. They don’t cost much and will turn the lights on and off automatically, without any effort from you.

Fish swimming in green aquarium water

But it’s not just your aquarium lighting that could be responsible for your green water problem. Sunlight could also be the cause.

It can be easy to miss while you are at work, but during the day time, the sun can shine through your windows, basking your aquarium in light. It’s basically inviting green water into your aquarium!

This is why the ideal location to set up your fish tank is away from direct sunlight. If it’s too late for that, you have another option… Keep your curtains drawn!

2. Nutrient imbalance

Dosing a planted tank is a balancing act. The right amount of nutrients keep your plants happy and looking their best. But too much of one nutrient and not enough of another is all it takes to throw your tank into chaos.

You see, these nutrients are not just utilized by your plants but the algae too. Makes sense, right? I mean, algae is a plant after all.

So by using too much (or in some cases too little) of a nutrient, you could be accidentally encouraging your water to turn green.

If you dose your tank with nutrients, the first thing you should do is check your water parameters. If you notice anything unusual, such as excessive levels of CO2 or phosphates, take steps to not only lower these levels but prevent them from happening again.

Murky green water inside aquarium

3. Unmaintained aquarium

Whether you like it or not, regular maintenance is a big part of owning an aquarium. In fact, many of the problems that beginners face can be solved just be keeping a regular maintenance routine.

If you are not getting the basics right, like maintenance, then narrowing down the cause of your green aquarium water is going to be nearly impossible.

Don’t maintain your aquarium? It’s never too late to start! Here are some quick tips you can use to get your tank back on track.

  • Perform regular water changes
  • Clean your filter
  • Check your water quality (use an aquarium test kit!)
  • Clean your gravel with a good gravel vacuum
  • Remove uneaten fish food
  • Ensure your tank isn’t overstocked

How do you get rid of green aquarium water?

Before and after getting rid of green aquarium water problem

Obviously, the best solution to getting rid of green water is prevention, which I outlined earlier in this guide.

If you have tried everything and still can’t fix your green water problem, then it’s time to pull out the big guns.

Below, I show you four solutions that are commonly used to treat green aquarium water. Best of all, these products actually work!

1. UV sterilizer

AA Aquarium Green Killing Machine UV Sterilizer

This is actually my favorite tool for ridding your aquarium of that awful green tint. Not only is a UV sterilizer safe for fish, invertebrates and plants, but it requires no effort on your part – simply turn it on and forget about it.

As water flows through this device, it is exposed to UV light. This UV light kills any microscopic organisms that are floating in the water, such as bacterial blooms and phytoplankton.

In just a few days, the phytoplankton responsible for your green aquarium water will start to die off. The result? Crystal-clear aquarium water.

Who would have guessed that light could be such an effective green water treatment?

2. Water polishing pad

Water polishing pad for filtering green algae out of water

The phytoplankton that turn your water green are so small that your filter is unable to trap them. The tiny suckers keep slipping through.

Fortunately, with a simple modification, your filter can turn into a green-water trapping machine – all you need is a polishing pad.

A polishing pad is capable of filtering out even the tiniest particles from your aquarium water, leaving you with sparkling, crystal-clear water.

As you might have guessed, this makes a polishing pad the perfect tool for removing that green tint from your aquarium water.

Remember that you need to replace the filter as it traps and removes the phytoplankton from your aquarium, so make sure you have a couple on hand and swap them out as they clog.

3. Daphnia (Water flea)

Wouldn’t it be great if someone else could get rid of your green aquarium water for you? If you are nodding your head, then get yourself some Daphnia.

This tiny creature will happily eat the phytoplankton that is turning your water green. Buy them live either online or at your local fish store. One hundred Daphnia can quickly make a dent in even the murkiest green water. It’s natural, cheap and effective.

Your fish will also thank you for adding Daphnia to the aquarium. Many fish, such as betta and tetra, find Daphnia a delicious treat!

It is important that you correctly identify your green aquarium water as phytoplankton and not some similar algae… Daphnia do not eat other types of algae.

4. Chemical removal

API Algaefix to control and remove unwanted algae growth

Add a dose to your aquarium, and your green aquarium water will disappear, like magic.

Sounds too good to be true, right? Well, in a way, it is. Using chemicals is my least favorite solution to green water. It’s not because these products don’t work – they do. In fact, they work really well.

The problem is that what I keep in my tank will be very different to what you keep in yours. With such a wide variety of plants, fish, critters, not to mention different aquarium equipment, you just don’t know how these products will affect your tank.

You don’t have to go far to read hundreds of horror stories where aquarium owners blame algaecides for the death of their fish.

I would also add that in a rush to get rid of their green water, beginners tend to overdose with these chemical products. And this causes more problems than it solves.

Given that the other solutions are much safer, and just as affordable, chemical treatment of your green water should be used as a last resort.


Green foggy water inside aquarium

Did you know that some aquarium owners actually want green water?

Yep, the phytoplankton that turn water green are a commonly used supplement for reef tanks and are eaten by corals.

For the rest of us, green water is an unwanted pest that blocks our view. So, you want it gone!

Now that you know what it is, what causes it and how to get it under control, green water should no longer be a problem in your aquarium.

How did you get rid of the green water in your aquarium? Let me know in the comments below!

Ian Sterling

I've been keeping fish for over 30 years and currently have 4 different aquariums – it's an addiction. I'm here to teach you everything there is to know about fishkeeping.

I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.

Comments (67)

Tank info
1. 55 gal freshwater
2. Temp 78.3
3. Zero ammonia, nitrite; 140 nitrate
4. 12 hour fluorescent lite
5. Alot of fish really
6. HOB 200gph 2 powerheads for undergravel filter 100 gph each
7. 8-10 gal water change per week
How I got rid of green water
1. Changed 1 of two filter pads to keep some good bacteria in HOB
2. Used 10 gals of distilled water (.89 per gal) used water conditioner and salt prior putting water into tank
3. Water just cleaned upped nicely that day very quickly

Hi Brad,

Thanks for the update on your tank! Just to clarify, your nitrate levels were 140 ppm? That is ridiculously high. The excess nitrate was possibly being used as a food source for algae that turned your water green.

hi Brad-

I’ve been freshwater fish keeping for about 56 years & recently got a horrible GREEN ALGAE BLOOM I never use chemicals but I did manage to cure it after weeks of frustration , I blocked all light to my tank for 3 weeks it is now crystal clear…what caused it was, I’d accidentally replaced my fluorescent tube with a marine tube which was giving to much light.mick

Hi Michael,

Thanks for weighing in. That’s such an easy mistake to make. Hard to believe using a different tube was all it took. I’m glad it was an easy solution where you didn’t have to use chemicals!

I have this happening now and I’m curious on what to look for in a uv sterilizer and what is a good brand. I have a 40 gallon.

Hi John,

If you are located in America, the AA Aquarium Green Killing Machine would be my go-to recommendation for hobby tanks. It’s pretty fool proof.

Hi Wanda,

Something like this is the right size for a 20 gallon. The next size down, a 3 watt would be too small. You can leave it to run 24/7 or if you are energy conscious, every time you see green algae appear. Either or, it won’t harm your tank.

I had a problem with greenish water for over a month and was going to try a polishing pad as I did not want to use chemicals. Instead, I used an old thick terry cloth wash cloth and cut it up in squares and placed it between the foam insert and charcoal insert in my aquaclear. Works like a charm.

Hi Brian,

That’s pretty clever – A DIY polishing pad. My only concern with repeating this would be the dyes and other chemicals that these can contain leaching into the aquarium, particularly in tanks containing sensitive fish. But that’s fantastic that you came up with your own solution!

Hi Ian,
I thought about that. I used a white terry wash cloth washed in tank water and made sure to line dry it and never used dryer sheets on it. My tank is now crystal clear and the wash cloth is green.

Hi again, Brian.

That’s the kind of clever thinking and attention to detail I can get behind! I love it. You have me intrigued now, and would love to experiment with how effective it is. I think this is may be a viable option for those who do not have affordable polishing pads available. My preference at the moment is to buy a bulk slab and cut to size, it’s very affordable if you plan on being in the hobby for a long time. However, terry cloth would likely be easier to wash and reuse – my experience with fine polishing pads, especially those around 50 micron, is that they have to be disposed of once they clog. Have you tried washing and re-using your design yet?

Hi Ian,

I’m going to leave it in for a few days to see if it continues to filter or if will be chocked up by algae. Even if I can’t reuse the piece of wash cloth currently in the filter, there’s so much wash cloth left that I can probably get another 8 pieces out of it. The trick was to make sure there is enough fabric in between the 2 filter that water doesn’t go around the wash cloth. Cost wise if you go to a dollar store and buy a wash cloth for a dollar or so it will probably be extremely cost effective.

Hi Ian,
I decided to change the first piece of wash cloth I placed in the filter pack. It was white when I started and a dark green when removed. I washed it out and placed it back in the filter pack. After a few days the tank had a very slight cloudy appearance. I think the problem with reusing the piece of original cloth was so it was saturated with algae that it became the problem. The second piece I placed at the bottom of the filter pack and again it remove any existing algae. I don’t think reusing the fabric would be a good solution but using a piece of wash cloth is a very effective and cost efficient solution to getting rid of algae, I tested the chemistry of the tank before and after and it slightly reduce the nitrate levels not that they were high.

Hi Brian,

Thanks for following up with this. It seems that it faces a similar problem as filter floss in that due to the fine pores, they are neatly impossible to wash and re-use. If someone can’t track down affordable filter floss, then this seems like a great idea. Once again, thanks so much for sharing!

I have 150 gallon African cichlid tank I have never had a green algae bloom before the only thing I changed was my filtration. I had two emperor four hundreds and a marine land 220 canister. I switched to the fluval Fx6. The switch happened two months ago I am now plagued with green water. No changes on location of tank no changes on lighting Any thoughts? I’ve been an avid Aquarist for more than 20 years So I’m no stranger to the hobby this situation just blows me away

Hi Mike,

Unfortunately, this is a tricky one to diagnose, I would not expect the Fluval to be the cause, especially if it was two months ago. It’s likely you are think back to the most profound change made in the tank, but it’s likely something else that has happened since. If you remove the green algae using a uv sterilizer or diatom filter,and it doesn’t come back, then it was a one off coincidence. If it keeps appearing, then it has a food source that needs to be cut back.

My filter stopped working and I didn’t notice because of my castle bubbler. When I noticed it had maybe been 2/3 weeks I didn’t have money for a new filter at the time. I have a big tank so another 2 weeks went by. I did a 50% water change and fixed the filter. Still green. I’m about to buy a uv filter because I can’t find an algae clumper and don’t want to use chemicals. What do you recomend?

Hi Jay,

Have you tested your water with an aquarium test kit first? This will determine if there is an obvious imbalance that needs to be fixed – for instance, your tank might need to be cycled again. You might not need to buy a UV sterilizer if you can fix it manually – UV sterilizers are not exactly cheap.

If everything looks normal and you still want to go the UV Sterilizer route, the AA Aquarium Green Killing Machine is the most reliable in terms of price and performance. They have replacement bulbs sold separately and last for years.

Hi there Ian. I am a small time aquarium operator compared to you guys but I hope you can help me. I have a 2.5 gallon Fluval brand tank for one betta fish, one snail and a live plant. I have a biological filter that keeps all the N levels where they should be. I got the snail to help with the algae dust (the kind that sticks on surfaces and he does a good job with it). But the tank is overrun with floating green algae. I do a 50% water change / gravel vacuum every 7-10 days. I have added a scrubbing pad as you recommended to the filter area. I have moved the tank away from the window. Still green water forms in 2 days after change. Any suggestions? Would the UV filter you recommend be appropriate to use in my tiny tank? Thank you! I appreciate any feedback.

Hi Laura,

Just so we are on the same page, your ammonia, nitrite and nitrate levels all look good? I only ask as a two day build up is shockingly fast – the algae would have to be feasting on a good food source for it to take over your tabk so quickly – even though it sounds like you are doing everything right.

In your case, it’s possible you may need to buy a UV sterilizer as a last resort. While a UV sterilizer will certainly work, but it will also take up a chunk of room in your tiny tank. It’s possible that the UV sterilizer may kill it all outright and you might need to just use it this once (or on occasion in the future) or it’s possible that you’ll need to use it on an ongoing basis unfortunately, this entirely depends on the rate that the algae builds up.

I have a 55 gallon freshwater tank I have done it all from water changes to buying a uv filter. My tank is still green what can I do.

Hi Tammy,

Is the UV filter you purchased the correct size for your tank? If it isn’t then it might not treat the free-floating algea quickly enough to kill it off.

Just another few questions…

Does the UV filter work? It can be hard to tell since the bulb itself is covered and may not actually be lighting up. You might have a faulty unit
Be particularly careful here, you shouldn’t look at UV bulbs. Refer to the instructions for more info.

Next, is the algae free floating? UV filters and water change will only get rid of free floating algae. Algae that clings to surfaces needs alternate methods of removal.

Finally, what are your tank parameters – the most common reason algae appears is there is a food source for it. Are lights on 24/7, is direct sunlight hitting it? What are you ammonia, nitrite and nitrate levels?

Hi Shahid,

Thank you for the kind words, I hope you were able to fix your green water problem!

Hello Ian
I have 75 gallon tank and I just bought the green killer machine (24w up to 100gal) 5 days ago and the water is not getting worse and it’s getting clear only a bit.
Do you know after how many days the water on my tank would be clear? I have my lights on for 7 hours every day ( timer)
Thanks for support

Hi Fransisco,

If it’s exceptionally bad, I would perform daily or bi-daily 50% water changes alongside continuous UV treatment. This 1-2 punch should get rid of it, assuming all your water parameters are within your desired range (planted tanks often need extra balancing)

Hello Ian
Thanks for your quick response. I will follow your recommendations and keep you posted.

Thanks for the support

Good luck Francisco,

I have my fingers crossed you are able to get rid of this green menace!

Just a big thank you !
My husband has a 400l tank and this is the first time the green monster has popped in for an unwelcome visit .My hubby was at wits end and your advice and guidance has given him new hope

WOW. I have green water. I was told that my tank itself should be replaced I thought this was kind of strange. Now after reading about all the wonderful solutions i’ Thankful that I came across this sight. I like the solution of using filter pads first. Where are these sold?? Thanks much for being there

Hi George,

These are are sold online or at pet stores. They are also sometimes referred to as filter floss. If it states how fine it is on the packaging, you ideally want 50 micron although 100 micron will do, as these trap the finest pieces of debris and algae.

Thank you Ian. I purchased the filtering pads. The next day the pas was green. Should it be changed more often now. My 30 gal tank is not directly in the sunlight but close to a window. I’m also cutting back on the amount of food daily and tank light. Are the magnetic glass cleaners helpful. Thanks much

Hi Again george,

As long as your tank is not being hit with sunlight, near a window is fine.

Magnetic cleaners are only useful for algae that cling to the surface of your tank. If the water is green, then it’s free-floating algae, which doesn’t cling to the glass – a magnetic glass cleaner won’t help here. A magnetic glass cleaner is just an alternative for dipping a hand inside the tank and wiping it yourself.

On the pads, you remove them once they have clogged with green algae. Depending on the type, some can be rinsed and reinserted while others need to be replaced. I prefer to use filterfloss here as you can buy a whole heap of it for cheap.

Thanks much Ian. Just one more thing I failed to mention. I had recently purchased one of those digital lights that are submerged in the fish tank. I was using this as my light instead of the top outside bulb. Could this have contributed to my green water. is just great. Wouldn’t know what I would have done without you

Hi George,

I have personally never had the chance to experiment with submerged lights (I use the boring standard ones that sit above the tank) However, it possible that the light was too strong for your tank which can certainly accelerate algae growth. However, this is a problem with all aquarium lights, not just submergeable ones. The lights that sit above can at least be raised higher, which will weaken the output hitting your tank to an extent. If you don’t want to buy a new light, is there a way to diffuse the output? Say, run a layer of silicone over the top? I’m unsure if this would work, but it’s an option.

Hello. I am so surprised that no one has mentioned ALCURELF (treats up to 530 gallons) Our tank was so green you could barely see our huge turtle in our 100 gallon tank. We were having company and didn’t have time to change the water so my husband ran to the store and bought the above mentioned what I now call “magic potion” and added the correct amount of drops per gallon and the tank within hours turned crystal clear! It looked better then if he had just changed the water and filters in it! We were amazed! My friend (who just recently passed away RIP) had mentioned it to me and used it but we had forgotten about it until we really needed something.
But I can’t help to wonder whether this effected the turtle in anyway? She seems fine. Swimming, eating and acting just like her self. The man who owns the pet shop assured us there was no harm. It’s been 5 days and the water is still crystal clear! I wish I could post a picture. I was wondering if you ever heard of this product? We will only use once in awhile and still clean and maintain on a regular basis. It does state however “Use daily for a crystal clear aquarium” THIS STUFF IS TRULY AMAZING!!!!!!!

Hi Cheryl,

I completely agree with you that Acurel is an amazing product. In fact, it’s my most recommended water clarifier.

I fall into the camp that prefers to avoid adding chemicals to water where possible. Clarifier in particular can irritate sensitive fish. Since everyone has different fish and critters in their tank, I prefer to recommend solutions that I know won’t lead to further complications. Regardless, I am happy that you found a quick and effective solution and thank you so much for sharing your experience!

Thank you for this article! I have performed a change of water 2/3 and the water became even greener. I also used acurel and no help at all what would you recommend doing? Can change the whole entire water and gravel in the tank while the fish is put into something else?

Hi Ira,

Same recommendations I make in my article. Water changes with either a UV sterilizer or extra fine mechanical filtration (50 μm) should do the trick.

I have green water in my 50 gallon tank.

1) I have 2 huge algae eaters in my tank. Why aren’t they eating the algae? I give them a small wafer once a day.

2) Also the tank is in front of a window that gives plenty of light. However, I have an aquarium background picture that I would think would reduce the sunlight. Could the sunlight still be effecting the tank?

Finally, do you suggest using the uv sterilizer, polishing pad, and daphnia together at one time?

Hi Marcia,

Just because something is called an “algae eater” doesn’t mean it will eat all types of algae. There is also only so much algae an algae eater can eat. If it’s growing faster than they can eat it, then it won’t look like they are helping.

If direct sunlight is hitting it, then it’s possible. Placing a tank near a window also causes temperature fluctuations, which isn’t ideal.

A UV sterilizer and polishing pad is just fine, combined with water changes. I don’t believe you would see better results by adding daphnia. Most people just use a UV sterlizer water changes to great success.

Please Help!!
I have a ten gallon tank that has a small goldfish, tetra, and a algae eater. The water turns green only one day after changing water completely (which I know is not recommended but my only option with how bad my tank gets). I keep tank covered all day long and only turn lights on at night when my son goes to bed. In the morning it gets recovered. I feed once every two days and have aqua tech filter rated for 10 gallons. I have tried tetra algae control with no success at all. I really dont have any money to spend on uv filters right now but I also do see how that could even be the problem since it never gets sunlight anyway. I am at such a loss on this aquarium. Any advice is appreciated.

Thanks you

Hi Ryan,

As always, you should start by testing your water with an aquarium test kit. It’s basically impossible to keep fish without one – it will allow you to test water quality. I’d say about 90% of tank problems can be linked back to poor water quality. I personally recommend the API master test kit for beginners, it’s the most affordable, has all the basic tests and will last for years.

Wagering a guess, I’d say your tank is too small for what you have in it (gold fish need a LOT of room, more that 10 gallons)

Hi, I have an outdoor fish tank with glass to the front and open to the top. Only goldfish but great! I have a uv steriliser and the water is chrystal clear. The problem is that the glass and rocks and plants go slimy dark green in under a week! I have a large water pump too with a filter sending even more air bubbles cascading.
Th water gets very warm, but I cover the top with white material to deflect sunlight but everything inside has to be taken out weekly and scrubbed clean. How can I stop this please? Any ideas? Thanks.

Hi Brian,

If the water is crystal clear then it’s not green aquarium water. I have other algae guides on the site, perhaps you can match it to one of those?

Hi, I have an 80l tank with 6 WCMM, it has a Fluval u3 filter and sits at 24/25 degrees. My problem is green water. My light is on for no more than 10 hrs and I feed them every second day. What could be causing the green water and how can I get rid of it please?

Hi Karen,

Are your water parameters normal? Testing can reveal clues here.

As for getting rid of it, any steps outlined in the above guide will remove green aquarium water

Green water is not always a catastrophe. I had it in my garden pond, and it feeds perfectly my fry, my shrimps and my snails. It’s live water full of plankton.

Hi DD,

You make a good point. Many fishkeepers use this to feed fry (baby fish) as well. When it’s used this way, it’s referred to as infusoria.

Hi Cheryl,

It entirely depends on how well fed and the type of fish you have. It’s hard to say.

We have a new (approx 2 mo old 20 gal tank) that we think is still cycling. But I’m the past week we have developed quite green water. We’ve gone several 30% watt changes but it comes right back. We’ve been trying to limit the time the light is on, but haven’t seen much success. We don’t believe we’re overfeeding. If we use a uv sterilizer, won’t we kill the good bacteria we’re trying to get established? How do you use a uv sterilizer as part of a healthy tank with an established biological filter?

Hi E Shoch,

At 2 months, your tank probably should be very close to cycling, if it is. If you are still reading measurable nitrite and ammonia, and it doesn’t seem to be decreasing over the weeks, then that is cause for concern.

On the UV question, beneficial bacteria isn’t free-floating, it clings to surfaces. Once there is bacteria established in your filter, it will multiply there. This is why a UV filter doesn’t kill the beneficial bacteria.

I currently have a 37 gallon tank that is about 2 months old (cycling) and it is really green. Do I need to wait for the cycling to be finished or can I use the uv sterilizer or the pads right now?


Hi Mathew,

It depends show far along the cycle you are. If your beneficial bacteria are somewhat established, you could get by with a UV sterilizer – you see these bacteria are clingers, and shouldn’t enter the water column where the UV sterilizer will treat it, at least not to an extent where it will make a difference. However, if you are just starting the cycle, then it can slow it down.

Hello Ian. I took my three garden goldfishes indoors for the winter here in Sweden. Bougth a 50l tank, came with led light and pump. Its placed in a dark corner in a chilly room 15-16°celcius and I keep the light on for eight hours/day. The water is local, untreated groundwater. In two days it becomes really green.. I tried using rainwater too, and the same happens, tho in about a week. What to do? I don’t mind the green if the fishes are happy, but are they?

Hi Bianca,

The water you are using will likely have more of an impact than the algae.

Groundwater can contain all sorts of different minerals and nutrients that algae uses to eat. It can also vary dramatically in it’s hardness. I would recommend buying a test kit (you should already be using one for ammonia, nitrite and nitrate) for GH and KH. If it’s too high, it can cause issues.

Rainwater is not recommended as it is incredibly soft and can interfere with fishes ability to osmoregulate, which can stress and kill fish.

I own a 2 foot wide tank with 5 Comet Fishes. The Comet Fish look healthy and eat well but even tho we change our tank regularly, water still becomes murky. We feed them pellets every night and btw the tank is outside, but sunlight doesnt really sine on it too often so i dont know what to do.

Hi Anne,

If the tank is open topped and sun shines in it every now again while being outside, it’s a recipe for growing algae. Block all sunlight, get a lid and performing regular maintenance should fix the issue.

Hello. I have 10 gallon glow fish tank. It was ok for a long tim.e 1 month ago water turned green. I clean it every other week. Changed water, added conditioner and defogger. Nothing working.

Hi ,I’m new to keeping fish. I’ve had mine about a year now. They were fine up until about 4 months ago . The water is cloudy and the green algae is settling on the glass and filter again from last week I changed the water scrubbed all the pebbles cleaned out all the filters and glass . Put my filter care in and clean water treatment and its started to go cloudy and the green is settling again in the glass and filters .I dont have plants in mine as someone said it could be those causing it. They are not in direct sunlight . Just normal daylight. I dont really have anywhere dark to put them .

Hi Jen,

What are the results of your aquarium test kit for Ammonia, nitrite, nitrate and pH?

Leave a Reply

Your email address will not be published. Required fields are marked *