What’s that bad smell?
**Sniff** **Sniff**
Is it coming from your aquarium?
If it is, it could be a problem…
Now don’t get me wrong, a healthy fish tank will have a slight smell.
No, it isn’t a fishy smell. Rather, a pleasant smell. Here are how fellow fish keepers describe the scent of their tanks…[1]
- Freshwater tanks – A mild earthy aroma, like freshly tilled soil
- Saltwater tanks – A slight ocean scent, like the beach
Hardly what you would call bad smells, right? I mean, you can buy candles with those scents!
The thing I must stress is that these scents are not strong. You should never close your eyes and guess that there is a fish tank in the area.
If you can smell your fish tank from across the room, and it stinks, there could be something seriously wrong with your aquarium.
Today, I am going to take you through all the common causes of these smells as well as how to remove any overpowering odors from your aquarium.
Contents
Causes of bad smells
How to remove bad smells
So, your fish tank smells funny, huh? Well, chances are that one of the following 7 causes is the culprit…
Causes of bad smells
1. Something died
I hate to break it to you, but that foul smell coming from your fish tank could be the remains of a much-loved fish. A rotting fish stinks and can fill your room with a horrible odor.
If you have a tank full of decorations or plants, a dead fish can be difficult to spot. And if you have a crowded community tank, a missing fish can easily go unnoticed.
Heck, I once had a common pleco that was shy and only came out at night. He spent his time hidden away in the corner of the tank. Days went by without me seeing him.
Well, he gave up the ghost, and I was none the wiser until a week later, when I noticed a musty smell coming from the tank. Upon investigation, I discovered his decomposing body deep inside his favorite hiding spot.
So now, it’s time for some fish roll call. Any missing fish could very well be the source of the strange smell. Be sure to check behind every rock, decoration and plant. You would be amazed at the tight spaces that fish can squeeze themselves into.
Your dead fish might not even be in the tank at all! This is common with jumping fish, like killifish, who can easily leap out of aquariums that don’t have a lid. Needless to say, a fish out of water won’t last long and will soon begin to stink.
If your fish can jump, check behind your aquarium stand and any nearby furniture to find the body.
Fortunately, a dead fish is an easy smell to control. The smell should soon disappear when the fish is removed.
How to prevent bad smells caused by dead fish
Unfortunately, this is the most difficult type of smell to control – no fish lives forever. It’s sad but true.
The best thing you can do is give your fish a long and healthy life by getting the essentials right. Good water quality, not overstocking, proper diet and regular maintenance go a long way to seeing your fish live a full life.
2. Overfeeding
I know you don’t want your fish to starve, but overfeeding is perhaps one of the worst things you can do to your fish. It can also be responsible for the horrible smell coming from your aquarium.
You see… Your fish are only going to eat so much food in one feeding session. Anything that is uneaten falls to the bottom of your aquarium and begins to rot.
As the food breaks down, it releases gasses, causing a foul odor. The smell becomes stronger as this uneaten food builds up.
A good tank maintenance routine, such as filter cleaning and using a gravel vac, should remove most traces of decaying fish food and the smell along with it.
How to prevent bad smells caused by too much fish food
It’s actually very simple – only give your fish the amount of food they eat in a single session. This way there will be no fish food left to give off bad smells.
Figuring out just how much requires trial and error on your part – with so many different species of fish and different types of fish food, the exact amount can vary dramatically.
A rule of thumb is to feed your fish only as much as they can eat in 5 minutes.
3. Fish poop
That bad smell could also be caused by the food that your fish do eat.
If your tank is stocked correctly, you will perform your weekly maintenance routine long before the poop begins to stink up your tank.
As you are likely aware:
More fish = More poop
The built-up poop quickly breaks down. As it does, it gives off a foul odor.
How to prevent bad smells caused by fish poop
This one is an easy fix! Don’t overstock your fish tank and only keep fish that are appropriate for your sized tank.
Overstocking is a problem commonly experienced by beginners who do not know better. Before you buy fish for your aquarium, use a stock calculator like this one or ask an expert for their opinion.
If it’s too late for that and your tank is currently overstocked, you have two options:
- Buy a larger tank
- Remove some of your fish
4. Rotting plants
While it’s less common than the other causes on this list, a rotting plant can give off one heck of a stench if left unattended.
Dead plants are easy to spot. They no longer look like themselves and often turn a slimy brown or black color.
You should remove dead plants anyway because they also foul up the water quality.
If you have a plant that only has a few dead leaves, some light pruning is all it takes to remove the rotting bits.
And remember, algae are also plants. Algae can rot, leaving you with a horrible musty odor. Blue-green algae in particular can make your aquarium smell funky.
How to prevent bad smells caused by dead plants
Keep them alive! Living plants don’t give off bad smells.
I know, I know… This is easier said than done. But by ensuring the plants have adequate light and nutrients, you give them the best chance to thrive in your aquarium.
Otherwise, why not invest in a clean-up crew for your tank? Malaysian trumpet snails happily devour any dead or decaying plants without any effort on your part.
5. Dirty filter
You know all the things mentioned so far? Well, every one of them can be sucked into your filter system.
In fact, it’s not uncommon to find more rotting sludge in the filter than in the aquarium itself. After all, the job of a mechanical filter is to gather any gunk that is floating through your aquarium and collect it in one place.
And because of this, filters can smell bad. Really bad!
How to prevent bad smells caused by your aquarium filter
I’ll be straight up – cleaning your filter should already be a part of your regular maintenance routine…
When the time comes to do a water change, rinse the filter media in the water you removed so that you don’t kill any good bacteria that are living on it. Doing so should remove most of the stinky sludge that has been stinking up the joint.
6. Your substrate
If the odor coming from your fish tank smells like rotten eggs, then your substrate might be at fault.
This concept is quite complex, so here is the beginner-friendly explanation:
Sand or very fine gravel that has compacted over time can produce zones where there is no oxygen. As waste gets trapped in these pockets, bacteria convert it into gases. Smelly gasses!
This gas then rises up through the aquarium and straight into your nostrils!
How to prevent your substrate from releasing bad smells
Once again, maintenance is the name of the game.
A deep gravel vacuum or regularly stirring sand may prevent these smelly pockets of gas from building up in your aquarium.
Before you select a substrate for your aquarium, read up on how to maintain it!
7. Water conditioner
The water conditioner you add during your water change could also be responsible for making your tank smell funny.
Let’s take my favorite water conditioner Seachem Prime. There is no mistaking the eggy smell it gives off when you open the cap on the bottle.
However, once you add the conditioner to your water, the smell soon fades to the point of being unnoticeable.
How to prevent bad smells caused by water conditioner
Avoid water conditioners with smelly ingredients like sulfur, which is often the cause of that rotten egg smell.
How to remove bad smells
Important: Before you read this step, make sure you have identified the cause of your bad smell from the list above. If you do not remove the cause, the following methods will not fix the lingering odor.
Step 1: Maintenance
First, you want to clean your tank. Your aim is to remove every piece of rotting organic matter in your aquarium.
When trying to remove bad smells, I highly recommend being more thorough than during a regular maintenance routine. So, set aside extra time to do the following:
- Wipe down the glass
- Clean your substrate
- Prune any dead leaves off plants
- Clean any rocks or decorations
- Clean your filter inflow and outlet
- Perform a partial water change
- Rinse your filter to remove trapped gunk
… And any other maintenance steps unique to your tank. If it looks like rotting gunk, and it doesn’t belong in your tank, get it out!
If you have not performed maintenance in some time, it’s very likely that your tank will smell worse after you clean it.
The reason for this is that you are dislodging all the rotting gunk, allowing it to float freely in the water. This should only be temporary. By monitoring your water quality with a test kit and sticking to a strict maintenance schedule, you should knock out the bad smells in no time.
But to speed the process up…
Step 2: Daily water changes
Over the next few days, perform a 10-15% water change. This will add fresh, odorless water to your aquarium while removing some of the foul smelling water.
Step 3: Add a carbon filter (optional)
While you are waiting for the bad smell to clear up, you can add a special ingredient to your filter to stop the bad smells in their tracks…
When it comes to absorbing bad smells, activated carbon works wonders. Also known as a carbon filter, this magical substance pulls odors from your water and stops them from reaching your nose – it will also make your water clearer, removing discoloration.
The only downside is that a carbon filter needs to be regularly replaced, at least once every month. Once the carbon has absorbed all it can hold, the bad smell will return.
Check out our detailed guide on activated carbon filters for more information.
Conclusion
If you have made it this far, you might have noticed a pattern to preventing bad smells in your aquarium.
Did you notice it?
That’s right…
Performing proper and regular maintenance!
You see, almost all bad smells come from rotting organic matter. And with proper maintenance, these should be removed long before they build up and stink up your room.
So, do yourself a favor and set up a maintenance routine. Your nose will thank you for it!
Got a tip for getting rid of bad smells? Let me know in the comments below!
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.
I have a 10 gallon tank with 6 small (about 3 months old) guppies and 2 small…”moss” balls. I believe the tank is fully cycled, the ammonia/nitrite levels always test at zero with my API kit. The tank has been up and running since around 10/1/18, so about 4.5 months at this point. I used Seachem Stability and Prime in the beginning while cycling.
The issue I have is the tank has a slight musty, almost mildew type smell. I really only notice it when I open the lid but it just seems really weird. I also have a 28 gallon bow front tank and that tank has no odor at all, which makes the odor in the 10 gal seem more odd. The water is nice and clear, I haven’t seen any algae in the tank, the fish are acting happy, so it seems like almost everything is fine… it’s just this weird musty smell. I’ve done partial water changes (up to 30%) and it doesn’t change the odor at all. I’ve also washed the lid and left it outside to dry in the sun just in case the smell was coming from the lid. That didn’t help either. It’s getting frustrating trying to figure out where this smell could be coming from since everything looks clean.
I only turn the light on for about 6 hrs a day and haven’t seen much, if any, leftover food in the tank. I only feed them a tiny bit of food once a day and probably miss a day here and there. But the fish don’t seem to mind. They don’t even seem to mind the smell so maybe I shouldn’t worry about it, but it just smells gross to me. I was hoping someone might have a suggestion about what could cause a smell like this and what I might be able to try to remove the smell.
Thanks for any help/suggestions!
Hi Kristen,
If you look at your filter is it overly gunky? Or maybe your gravel needs to be vacuumed. Over time grime and gunk can build up here and can give off a smell.
I was given a 30 gallon aquarium. The water is very cloudy and smelly. I’m ready to clean the tan the tank again plus throw away the plants that were in there because they’re almost black and my tank is smelly. MY question is can I put real small seashells and mardi gra beads in bottom of tank for more color?
Hi Ellen,
Real seashells are going to rot in freshwater and make a bad smell. You would have to really clean them thoroughly first. I would suggest sticking to aquariums safe pebbles – these come in a wide range of colors.
My aquarium has been properly cleaned out as my shubunkin died of ich 😖 and I used boiling hot water to clean everything and to kill the ich. After I cleaned everything and cleaned my tank I put everything back in and fresh water with conditioner, I left my filter running as might be getting another fish Saturday (black moor) but a strange smell is coming from my aquarium and idk what it is…. Please help
Hi Becky,
Have you tested your aquarium with a test kit? What does it read for ammonia, nitrates and nitrites? If you cleaned everything, then you’ll need to cycle your tank all over again here is a guide on how to cycle your tank. If you don’t cycle your tank, your next fish could get sick too.
Hi Ian!
I just got my betta fish two days ago. Today, the water became very cloudy and began smelling bad. I definitely over-fed the fish the first time because some food fell to the bottom of the tank. However, I read that the cloudy water could be due to microorganisms establishing themselves in the tank since it has just been set up. Today I did a 30% water change and filtered some of the gravel to get ride of any excess food sitting in it. The water is still cloudy, but I’m not too worried about it because I think it is just the tank cycling. However, the smell has not gone away so I’m considering changing the filter and putting activated carbon in the filter as well–is this the right next step?
Hi Grace,
It’s generally recommended that you cycle your tank before adding fish. Cycling is with a fish in the tank is a rough process and can cause fish to become sick or even die.
If I had to guess, did you buy your fish and setup from a big name petstore? Unfortunately, these places often skip telling you all the essential information. It’s not the employees fault, it’s just the training materials provided by management are not satisfactory.
I suggest reading up on fish-in cycling to give you a better idea of how beneficial bacteria establish themselves in your filter. You should NOT change your filter since this is where the bacteria live. Instead, rinse it in freshwater. Those disposable filter cartridges are terrible for the hobby since they actually encourage this to happen. Activated carbon is very effective at removing bad smells but it will not prevent the cause. It will also need to be regularly replaced as it can only absorb so much before becoming “full”
Have you tested your water with an aquarium test kit? This can provide clues as to why your tank stinks. You should already be doing this daily while cycling anyway!
Hi Ian,
Thank you for sharing all this information. I’m looking for some advice.
I came back home today to see that my blood parrots have developed some white fluffy hairy (not cottony) stuff on their fins. I have a balashark and a sailfin pleco too in the same tank and they don’t seem affected at the moment.
I dont know what the issue is, although i noticed a piece of driftwood had quite a lot of white cottony fungus on it. Its been in the tank for 6months now but I’ve taken that piece out. I’ve put the other piece of drift wood and lavarocks back in.
The water also smelled musty and moldy. I’ve done a 50% water change and bumped up the temp to 30. I also dosed the tank with seachem paraguard. Im not sure what else i must do.
Please advice. Is this a fungal infection or is this something else? How to I save my fish?
Hi Kmenon
Unfortunately, without seeing your fish with good quality photos, diseases and infections are almost impossible to identify with just a description.
I’d suggest posting this to an online aquarium forum. Simply sign up and post a pic with your tank parameters, and others will help you diagnose what is going on with your fish. I sometimes do this myself when I want a second opinion!
Water clear no food debri any where fish healthy But still has slight pond smell, filter clean fish tank clean just a faint smell reminds me of a pond. ???? Otherwise everything excellent…..
Thank you…..
Hi Dale,
There isn’t really enough information to go off here. Can you smell it when you stick your head over your tank or from the other side of your room? If it’s the first one, it shouldn’t be an issue. Also, have you tested your water parameters? Is it a soiled tank? How often do you clean your tank Do you have too many fish? Etc. These things are easy to diagnose in person, but online it can be darn near impossible.
Ian I really need your advice. My son has a 55 gallon tank that had a massive algae bloom. We had been treating it with Prime but was told to use Algaefix to control the algae. It worked!!! However we have still been treating it and I fear that maybe we don’t need to use the Prime anymore. The filters are being changed when the indicator says to(about a week). He did have two fish die unrelated to this. His plaeco ate them. We are now left with a musty mildew smell and I’m afraid we are all gonna get sick. Last week we changed out half of the water. One side of the tank is in front of a window but he has a black out sticker on the back and we cover the side that’s infront of the window. How do I get this smell out? Please help!!!
Hi Myria,
Seachem prime, among other things, dechlorinates your tapwater. It will have no impact on algae. If you are adding in tapwater for your water change then you absolutely need to use prime – it will kill your fish otherwise.
What does your aquarium test kit say? This should clue you in as to what is happening in your aquarium.
If the algae bloom was white and turned your aquarium cloudy then it’s a natural part of the cycling process. If you have never cycled your tank then that is more than likely the reason why your fish have died. If you don’t know what I am talking about here, please read my beginners guide to the aquarium cycle. It is perhaps the most important part of keeping fish.
As for changing your filter pads once a week, that’s overkill and is possibly partly responsible for your tank problems. Grab a filter that doesn’t force you to use those nasty “disposable” cartridges, it wills ave you a fortune in the long run.
Even if fish die, it shouldn’t take much effort to remove the bad smell left behind.
I just set up a new 75 gallon tank. I only have 5 koi fish in it that are about 5 inches in size each. I feed them once a day. I have a sun sun 304b canister filter with carbon, ceramics, and bio balls. The tank smells kinda like broccoli. Its starting to smell up my living room. Ive done water changes and vaccuumed the gravel and it still smells. Any suggestions? I am going to try cleaning the intake and output hoses to see if that helps.
Hi Steven,
Firstly, It’s also possible that something in your new setup is giving off a funny smell which may disappear in time – cleaning pipes and equipment with vinegar before rinsing with freshwater may help.
However, I am concerned about the number of Koi you have in your tank…
That sounds like too many koi for a 75 gallon tank, especially for that size. It’s generally recommended that 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.
It’s possible that overstocking the tank is causing waste to quickly accumulate, causing the stench.
I just started a 39gal tall cichlid tank. I have 5. I was wondering what the clear junk that comes out of nowhere is exactly. I do a partial water change almost daily because it’s bare bottom. I have a good canister filter, but the poop just sits at the bottom. I’m forever scooping it up. Ugh! But, that clear gunk is driving me even more nuts. Help!
Thanks
Hi Sheryl,
Could it be a biofilm that is feeding off nutrients in your tank?
I found this cool idea how to make a aquarium out of a ice tea jug with the spout to remove water. I have 1 beta and 2 miniature African frogs I know I put in one to many first the tank was cloudy went to petsmart and they told me to get a Moss ball put it in the tank and it cleared up but the next day it smelled like something was died in the 2 gallon tank. I have no filter or air pump. I have to keep a lid on the container because I have a cat and if he had his way he would be eating my beta. I have fine sand at the bottom and small rocks on top of that. I want to remove the sand and just keep white rocks. First should I get a air pump? and can I remove my plants out and all the fish into another container with the old water to remove the sand I don’t want it in there and I have 2 live plants. We have no room for a bigger aquarium and this one seemed to fit perfect on the end table. One problem the cat I have to lock him up in the bedroom while doing anything with the tank because he is wanting to get to the fish. The lip we have put lots of holes in it so all I want to do is start all over can I do that. I put out 2 gallons of water in a bucket yesterday with the lid on it because my plans where to take everything out of the jug remove the sand and put in the new rocks keeping the fish in the old water until I get the tank set up. I don’t have room for anything any bigger. this tank I just set up last Sunday. Gotta get rid of that smell.
Hi Maureen,
I’d say you are overstocked for a 2 gallon tank. It’s going to be hard work to maintain a tank this size – small aquariums require much more work than larger ones. Constant water testing and water changes will be key. If you are serious about this fish keeping thing then it will be easier to just get a larger tank.
It also sounds like you have not cycled your aquarium, an essential first step of fish keeping. White cloudy water is common on new tanks and generally clears up by itself – the marimo moss ball was unlikely to be the fix. I’d also suggest using an aquarium test kit, so that you can see what is happening in your water, it might give you clues as to why it smells.
Regarding your air pump question, if it’s for an air stone, betta prefer calm water.
I changed my filter today and part of my water, but used a water conditioner for the rest. I also changed from sand to gravel, due to a problem with algae growing in the sand. Since doing this, the room the tank is in smells like a permanent marker exploded. What could be the cause of this? Are my fish going to be ok?
Hi Carson,
It’s possible that when you changed the sand, it released pockets of nitrogen gas that were trapped inside, which could be responsible for this foul smell. If this is the case, air your room out – open a window and get a fan in there. If the smell doesn’t come back, then there shouldn’t be anything to worry about.
My aquarium smells like rotten and the water is very cloudy, but I only have one fish and he’s perfectly ok. I already changed the water once, but it didn’t solve the problem. Any ideas?
Hi Nancy,
Are you regularly cleaning and maintaining your aquarium? Start by testing your water with an aquarium test kit, just to make sure nothing funny is going on. After that, regularly maintaining your tank is vital. For instance poop and uneaten food will fall to the bottom of your aquarium and gunk it up, requiring you to clean it with a gravel vac. As for water changes, you should be doing them regularly, not just when your fish tank begins to stink.
What’s regularly? Once a week? Every two weeks? We have the same problem.
Hi Veronica
It all depends on the size of your tank, the type of fish, filter system etc. Using an aquarium test kit to keep an eye on the water quality will give you a better understanding of what is happening in your tank in regards to water quality and how often you should be changing it. Water changes are typically performed every one or two weeks, during this time most hobbyists clean their tank too.
OMG. It was my pleco who had died. I had not seen him for a week which isn’t unusual. He is super shy. When I cleaned my tank looking for the smell, I found his rotting body. It was disgusting. I removed the body and added some carbon as per your suggestion. The smell was gone in under an hour. Thank you.
Hi Steve,
Oh no, that sounds like an awful surprise. I’m sorry for your loss. I’m glad you were able to remove the stink though.
I wouldnt use certain ornaments as I once found the skeleton of my plec trapped inside starved to death!!