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Why Does My Fish Tank Smell? 7 Common Reasons Explained

Gregory Iacono

By Gregory Iacono


Having an aquarium or fish tank is a very popular hobby in the United States, with nearly 15 million households having at least one. Amazingly, in 2020, the aquarium market in the United States had a value of nearly $2.6 billion, making fish the 3rd most popular pet behind dogs and cats.

If you’re one of the millions of people who love colorful fish and fish tanks, there’s one common problem you might face: a fish tank that often smells awful. If that’s you and you’re looking for a solution to the smell, read on. Below, we’ve listed seven common reasons why your fish tank smells. Plus, we have other tips and advice to help you keep your tank and fish happy and healthy.

The 7 Common Reasons for a Smelly Fish Tank

1. You’re Overfeeding Your Fish

Image Credit: hedgehog94, Shutterstock

One of the most common mistakes fish tank owners make is overfeeding. Overfeeding is unhealthy for your fish; if enough uneaten food is collected in your fish tank, it will start to rot. Once that starts, the rotting food releases a gas that can cause a nasty smell to emanate from your tank. It’s best only to feed your finned friends as much as they can eat in 5 minutes. If food remains left over after 5 minutes, you’ve given your fish too much. If the food is all gone in 3 minutes, give your fish a little more!

2. You Have Too Many Fish, and It’s Overwhelming Your Filtration System

When you have too many fish, it can easily overwhelm even the best filtration system. The more fish you have, the more poop they will make. There are two simple solutions to this problem. The first is buying a larger tank to keep all your fish. The second is to remove some fish. You can put them in another tank or give them to a friend.

3. Your Filter Is Extremely Dirty

Hands comparing old and new activated carbon material of a fish tank filter
Image Credit: Ladanifer, Shutterstock

Another common reason for a foul odor coming from your fish tank is that the filter is filthy. A dirty, sludge-filled filter can’t correctly process all the waste out of your water, including urea from fish urine and other foul-smelling debris. In some cases, the filter can have more fish excrement and rotting food than the tank itself. The obvious solution is to make cleaning your fish tank filter a habit. Experts recommend cleaning your tank filter once a month, but if you have several fish, once every 2 or 3 weeks might be a better choice.

4. Your Fish Tank Has Plants That Are Rotting

Usually, dead plants in a fish tank are easy to spot. They lose their beautiful, dark green color and start looking brown, black, and, sometimes, slimy. If that happens, you must remove the dead plants quickly. The longer they’re left in the tank, the more they will rot and produce gasses that stink.

Another solution is to prune your plants if only a few leaves have begun to rot. Lastly, removing algae from your fish tank should be something you do regularly. Remember, algae is also a plant and, if it rots, will cause the same stinky problem as other plants.

5. One or More Fish Are Dead in Your Fish Tank

Dead yellow fish in home aquarium with its belly up. Horizontal color photography
Image Credit: Andrii Oleksiienko, Shutterstock

Like dead plants, dead fish are usually easy to spot in a fish tank. On the other hand, depending on how many plants and decorations you have in your tank, a dead fish might sink somewhere and be hidden for days or even weeks. If that happens, you can bet that a nasty smell will start coming from your tank.

It’s worth noting that a dead fish that has jumped out of your tank and fallen behind it will smell even worse. In other words, be sure to check inside and outside your fish tank if there’s a smell coming from it and you don’t know why.

6. An Ingredient in the Water Conditioner You’re Using Is the Problem

If you’re like many fish tank owners, you put water conditioner into your fish tank’s water every time you clean it. However, one problem with water conditioners is that many are made with sulfur and can smell very much like rotten eggs. The good news is that the smell usually disappears after a few minutes. If you don’t enjoy it, you should consider changing the water conditioner you’re using.

7. Old, Compacted Substrate in Your Fish Tank Is the Culprit

betta fish swimming near the substrate in the aquarium
Image Credit: Andrej Jakubik, Shutterstock

Substrate refers to the material you use at the bottom of your fish tank, whether sand, pebbles, stones, crushed coral (in saltwater tanks), or a combination. With enough time, the substrate you use can become compacted, and when it does, zones emerge where no oxygen is present. These “dead zones” will start collecting bacteria which will inevitably turn into gas.

How To Prevent Your Fish Tank from Smelling

You can do several things to prevent your fish tank from smelling, some of which we’ve mentioned above. Your goal should be to remove any organic matter from your aquarium that will inevitably rot and produce gas in your tank. Below are a few methods to do just that and keep your fish tank clean and pristine.

  • Change between 15% of the water in your fish tank every week. If your tank has several fish, 25% of the water should be changed instead.
  • Clean and replace your substrate occasionally. Most fish tank owners do this every 3rd time they clean their tank and filter.
  • Stir up the substrate to release waste, gas, and other debris so the filter can grab it.
  • Clean your fish tank’s filter, including the inlet and outlet. Experienced aquarists suggest cleaning the filter once a month.
  • Use a cleaning device to clean algae off your tank’s sides whenever necessary.
  • Prune or remove any dead or dying plants in your tank.
  • Use activated carbon in your filter, or buy a carbon filter. A carbon filter is one of the best ways to reduce smells in your tank. The only downside is that carbon filters need to be replaced often. For most fish tank owners, it’s an acceptable cost to ensure their tank stays pristine.

Final Thoughts

Most odors emanating from fish tanks are preventable or require a relatively quick and easy fix. Removing dead plants and fish and not overfeeding your fish are common solutions to resolve lingering odors. When it boils right down to it, the best way to keep your fish tank from smelling is simply routine maintenance. One thing is certain; the better your fish tank maintenance skills, the fewer problems you’ll have with nasty smells.

Featured Image Credit: Frantisek Czanner, Shutterstock

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