Why Does My Fish Tank Smell? Five Stinky Reasons and How to Fix Them

Why does my fish tank smell? That’s a good question.  If you have a fish tank, there’s a good chance you have encountered a strange smell in or around it.

No matter how much you care for it, bad smells can happen. It happens to all of us. That being said, there are many reasons your fish tank might smell, and none of them are good.

A smelly tank can be more than just unpleasant and embarrassing; it can be dangerous for your fish. If there is a bad smell in the tank, it means something isn’t right. The longer you wait to fix it, the worse it will become, and the more likely your fish is to become ill or die.

Common reasons for a tank to smell include dead animals and plants, a filter that isn’t working correctly, an overpopulated tank, or a filter that is overwhelmed and can’t keep up with the bioload.

You don’t have to be an expert in the aquarium industry to fix your smelly tank. You just need to have some patience and the right methods to correct the problem. 

Five Reasons Your Fish Tank Might Smell and How to Fix It

Here’s what I found while researching reasons your fish thank might smell. It might be an obvious reason, or you may have to take the time to examine your tank closely to find out the exact cause.

If the smell is really severe and you attempt to fix one problem and don’t see any improvement, there may be another cause. 

It’s not uncommon for a tank to smell bad for more than one reason. If you are asking yourself, “Why does my fish tank smell?” It could be one of these reasons.

Dirty Filter

It’s the filter’s job to remove nasty, smelly things from your fish tank.  As the water flows through the filter, it collects anything that doesn’t belong in the water so clean water flows back into the tank. Even though this is what your filter is meant to do, it can become stinky and bad itself. 

If you notice a smell that comes from the surface of your tank, check to see if it’s the filter. You can remove the cartridge and sniff it as well.  Filters that smell bad are likely full and no longer functioning as they should.

How to Fix It

Many filters have cartridges that can be removed and rinsed thoroughly before being replaced. You should be able to get rid of the smell by rinsing them. If that doesn’t work, you may just need to replace them. 

If your filters are not reusable, you should just replace them once you notice they are starting to smell bad. It’s a good idea to remove the entire filtration system and clean it thoroughly before adding the new filter cartridge and restarting it.

Dead Fish

Dead fish can cause a tank to smell bad if they are left in for too long. While many fish float when they are dead, some don’t make it to the surface. Some fish get stuck to the filter or under rocks in the bottom of the tank. 

dead fish in fish tank

If you have a lot of plants or decorations in your tank, a dead fish could be stuck or hidden under or behind them as well. As the fish decomposes, it will cause a bad smell in the tank. Never leave a dead fish in your tank, whether there is a bad smell or not.

How to Fix It

You should remove any dead fish in your tank as soon as possible. Use a net or small cup to scoop them out. If you suspect there is a dead fish in the tank, but you can’t find it, take a small net and scoop through the tank. 

This will stir up the water and may help bring any dead fish to the surface so you can easily see them and remove them. You can also try turning up the flow of the filter so it can churn the water too.


Overfeeding in fish tank

A lot of people overfeed their fish and don’t realize it. When you overfeed fish, they get more food than they can eat. The uneaten food can sink to the bottom of the tank, settle on plants and gravel, or get sucked into the filter. It can breakdown and decompose, causing a bad smell in the tank. 

Overfeeding can also cause your fish to produce for poop and even become sick. It’s also important to make sure you are feeding your fish the right type of food. If the fish aren’t eating it, it may be more likely to pollute the tank.

The wrong food can also cause digestion problems in fish, which can also lead to more pollution in the tank.

How to Fix It 

Watch the fish eat when you feed them. Learn how much food the fish eat in one feeding and only add that amount each time. If you do happen to add too much food to the tank, remove any food that goes uneaten after the fish are finished.

This should prevent the tank from smelling badly in the future. If you feel there may be food in the tank or filter, try to remove it to help get rid of the smell faster.

Dead Plants

dead plants in aquarium

Plants add a lot of nutrient to an aquarium and make your fish happier. They can also cause bad smells. Plants are always growing and regenerating, but some parts of them also die in the process.

The dead parts usually fall to the bottom of the tank where they decompose or get stuck in the filter. 

As the dead plants break down, they release gasses into the tank. When the fish move the water around or as it passes through the filter, these gases and bad smell are released, and you may notice them.

How to Fix It 

Keep an eye on your plants and try to remove any dead stems or leaves as they fall off the plant. If you see leaves in the bottom of the tank or stuck to the filter, use a net to scoop them out. Also, make sure you do not have too many plants in your tank. 

An overcrowded tank can cause a variety of problems, including bad smells. It’s a good idea to have a mix of different types of plants to help add oxygen to the tank without taking over.

Fish Poop

fish poop in tank

Fish poop like every other living thing and if there’s a lot of it, it will cause a bad smell in the tank. Too many fish in your tank can lead to too much poop. Overfeeding can also cause the fish to produce more waste than normal.

Your filter can help get rid of some of the fish poop, but if there are too many fish, the filter may not be able to keep up with all their poop. This can cause the filter to smell or the entire tank to stink.

How to Fix It 

Check to make sure your tank is not overloaded with fish and that the filter is working properly. If the tank looks dirty and smells bad, it may be overpopulated. If you are not sure how many fish can safely live in your tank, talk to someone at your local pet store, or do some research online. 

There are a lot of things that determine the right amount of fish for a tank. Some fish like goldfish produce a lot or waste and ammonia than other fish, which can lead to a bad smell. In most cases, the size of the fish and the size of the tank determine the number of fish that can live it in.

If you think overpopulation or too must waste may be the problem, remove some of the fish and allow the tank to cycle for a few days before checking for improvement. You can move the fish to another tank or sell them. You can also do a water change. 

This will allow you to remove some of the dirty, stinky water and replace it with clean water. This will also make it easier for the filter to clean the tank. If you are using water from your tap, make sure you add dechlorinating tablets to the tank to ensure it doesn’t harm or kill the fish.


How to get rid of fish tank smell

There are a lot of things that can cause your fish tank to smell bad. If you are asking yourself, “Why does my fish tank smell?” you should consider some of the reasons pointed out above. 

Monitoring your tank will make it easier to find the problem and also help you take precautions to ensure that it doesn’t happen again.

Nobody likes a smelly tank, but it’s just part of being an aquarium owner. Most of the time, it’s an easy fix, and the smell goes away quickly.


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