Why My Fish Tank Smells? [With EASY Treatment Options]

While visiting a friend recently, I admired the array of fish in her aquarium. I noticed it smelled a little funky, though. I asked how often she has to clean it, and she admitted, “I try to clean it regularly, but there’s been this awful smell lately and I don’t know why.” To help my friend and anyone else with this problem, I looked into it and found possible causes, steps to determine the source, and solutions for getting–and keeping–the smell out.

Dead fish, uneaten food, and overcrowding are the most common causes of a smelly aquarium. It could also be a dead plant, a dirty filter, or an incorrect substance. These substances include the type of substrate, water conditioner, or other added chemicals.

For a dead fish, look under items that could be hiding it. If your tank is open, check the surrounding area–behind and next to the tank–for any jumpers. Looking under items, or in the substrate, can also help you find rotting food or plants.

Murky water could mean a dead plant or a dirty filter. Smelly water might also be from sulfur in your water conditioner.

Dispose of the source and clean the tank. For excess food or fish waste caught in the bottom layer, use a tank vacuum or scoop. If a few dead leaves are the culprit, just prune those leaves. Clean or replace a dirty filter.

To prevent future odors, check if you’re following guidelines for the number of fish, size of fish, and substrate that suits your aquarium. Feed the minimum amount and clean the tank regularly. Investing in a carbon filter or freshwater snails can make this easier.

Let’s get into more detail to help you get rid of that obnoxious odor.


Why Does My Fish Tank Smell?

Your fish tank could smell for a number of reasons, but the most common ones are dead fish, rotting uneaten food, and excess fish poop–likely from overcrowding.

1.Dead Fish

A dead fish starts to smell because of its chemical makeup. Essentially, without its living functions, a fish will get broken down by bacteria that turns these chemicals into worse-smelling ones. It may take a few days to notice this scent, but ignoring it will only make it worse. It could also put your other fish in danger.

2.Rotting Uneaten Food

Rotting uneaten food has a similar reason for smelling. Whatever your fish don’t eat falls to the bottom, also called substrate if your tank has a bottom layer. Just like a dead fish, bacteria break down this food and release odorous gasses.

The reason for this uneaten food could either be overfeeding or using the wrong food for your fish. The wrong food might be food that is simply too large for your fish.

3.Fish Poop

Another smell coming from your substrate could be fish poop. While this is a normal process,  too much accumulating too quickly will break down into foul smells. This is usually the result of overcrowding, or having too many fish in too small of a tank.

4.Dead Plants

Besides dead fish, dead plants could also be the cause. It may not be as likely as the ones above, but bacteria break these down, too. It could also be an indirect cause, as dying plants don’t release as much oxygen and this can eventually kill your fish.

5.Dirty Filter

The odor might also be from a part in your tank, like a dirty filter. A dirty filter can get clogged or backed-up, stopping it from doing its job. This can also make the bacteria problems that we’ve mentioned worse.


The second part of your tank to consider is your substrate, the material you use as the bottom layer. A thick layer means more places for food or feces to get stuck, but a finer substrate also means less oxygen. That’s where the bad bacteria and smell comes in. If you don’t have the right substrate for your tank–for all the fish and plants inside it–or you haven’t been cleaning it often enough, that could be the issue.

7.Water Conditioner

Another part of your tank maintenance to consider is your water conditioner. A nasty smell from your tank probably smells like ammonia or sulfur. So, if your water conditioner contains sulfur, it might be the culprit.

How to Determine the Source of the Smell

Determining the source of your tank’s odor just takes some digging. Unless you can see the source already, use a process of elimination.

First, check under and around items in your tank. Look for fish remains first. Some fish don’t come out often, so make sure you’re thorough. If you have an open top, check outside the tank for jumpers. While you look under your tank items, you may find pieces of dead plants or uneaten food. If a dead fish does not turn up, these rotting materials are probably your source. Rotting food can also get stuck in your substrate. If you find some under your tank items, chances are they might be stuck below it, too.

Next, stir the substrate around to see if anything stuck comes out. If rotting food doesn’t come out, but no other source can be found, the food might just be clinging on or stuck deeper in. Stirring it is also helpful for finding excess fish poop. If this isn’t obvious either, and you suspect your tank might be overcrowded, you can use this calculator to check.

Take a look at your tank water. If you didn’t find any plant remains under your tank items earlier, it might require examining the water overall. Dead or dying plants will typically look dark–a brown or black shade–and make your tank water cloudy and opaque. Algae might be a source, too, especially if you have blue-green algae or any in the green algae family, as these are breeding grounds for bacteria.

If your tank water is cloudy but it’s not from the plants, it could be your filter. A clogged filter can result in murky-looking water, so simply looking at the clarity of your water might be the answer.

How to Get Rid of Smell from Aquarium

You’ll want to get rid of the odor as soon as you find what it’s coming from. If you don’t, the smell could get worse.

  • Dead Fish: If the fish or its remains are in your tank, use a net to get it out. Make sure to sanitize the net after using it again. For disposal, you can either put it in a paper bag and throw it away, compost it, or bury it. If you choose the trash route, you may want to freeze the fish first to lessen the odor from your trash. If you choose the burial route, be sure to choose a dry spot and make your hole at least 2 feet deep.
  • Excess Food/Fish Poop: The focus here is on your substrate. You can use a scoop if the pieces are easily reached, but you may need to use a tank vacuum on it. You can clean the substrate by taking it out of the tank first or doing it underwater. Once the substrate is clean, you’ll need to clean the entire tank, including the filter.
  • Dead Plant: If only a few leaves are dying on a plant, you don’t have to throw the entire thing out. Simply prune the dead leaves and dispose of them. For an entirely rotting plant, pull it out and dispose of it. In either case, start water changes after to make sure the rotting parts didn’t leave anything behind.
  • Dirty Filter: Cleaning your filter should be a regular process, but it is a necessity in this case. First, fill a container with some of your tank water and unplug the filter. Separate the sponge or insert from your filter. Put the filter in regular water and the sponge/insert in the tank water you took out. Use this to rinse the sponge or insert to keep the good bacteria in it. You can use a razor blade or similar tool to get any algae or deposits off your filter unit. Then, rinse it in regular water. Put the sponge or insert back into your unit and put some of your tank water into it. Then you can plug the filter back in.

Once you’ve taken the necessary steps for removal, do a smell test after a few hours to make sure you identified the right source and taken care of it.

Steps to Keep Your Tank Smelling Fresh

Once you’ve eliminated the odor, here are some tips to keep any odors from coming back:

Keep a headcount of your fish. Whether you’re the type of owner to name and check in on each of your fish, keeping an inventory of them is key. Regularly count them, and if some are missing, count at a different time of day to see if they were hiding before.

Feed your fish the minimum amount. Feeding too much presents more dangers to them than feeding too little. However much food your fish eat within 5 minutes is your best bet. The type and size of food will vary based on the species.

Don’t keep too many fish in too small of a tank. If overcrowding is an issue, you will either have to find a new home for some or find a bigger tank. It’s important to note the right size tank for your size of fish, too.

Feed your plants, too. Giving your plants enough light and the right soil is important to your aquarium’s ecosystem. If you find their leaves dying and falling off frequently, you can add snails to handle some of this for you.

Schedule your cleaning routine. You don’t have to do every part at once–in fact, you probably shouldn’t–but schedule time for each step weekly or monthly. Filters should be cleaned, on average, once a month. If you add an activated carbon media to your filter, this can help to absorb odors, but it needs to be replaced more often. Regularly clean your tank by wiping it down, rinsing the substrate, and rinsing the filter. The minimum water changes you should do are between 10-15% once or twice a month, but it’s better to do a 15% change once a week.

A bad smell from your fish tank is an issue for both you and your fish. It might be from something rotting or a build-up of waste. Regardless of the source, it’s important to pay attention to any and all changes in your tank and keep up regular maintenance. Your tank–and your house–will be a better place for it. Have you ever dealt with this problem? Was the source something we didn’t mention? Comment below!

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