is supported by our readers. When you buy through links on our site, we may earn a commission. Learn more.

What To Do If Your Fish Tank Smells Bad: A Complete Guide

Does your fish tank smell bad? If so you have a problem. An aquarium should never smell foul as that’s a sign of rot and decay.

Your fish are in an unhealthy environment and if they die they will further pollute the water, compounding the issue.

Let’s look into some causes and fixes for smelly aquarium water!

Why Does My Fish Tank Smell Bad?

Here are a few common reasons why you fish tank may smell bad:

Check for Dead or Decaying Inhabitants

One of the most common reasons fish tanks smell bad is because one or more of the inhabitants have died. If you don’t get to them within a few hours intestinal and environmental bacteria and fungi begin breaking down the fish.

They release ammonia and sulfurous compounds that make an unmistakable odor. Since dead fish tend to float due to their swim bladders and decomposition gases they should be easy to find and remove.

A few hours after removing the rotting fish, sniff your tank water for foul odors and check the water quality using aquarium test kits (see below). Water changes are almost always a good idea if a fish dies in mysterious circumstances but these tests will help narrow down the causes.

Perform Tests on the Water Chemistry

Testing the parameters of your aquarium water will go a long way towards solving the mystery of a stinky fish tank!

A sure sign that leftover food or a missing fish is rotting is a spike in Ammonia or Nitrite. In a healthy aquarium your Nitrifying bacteria are hard at work breaking down Ammonia into the less toxic Nitrite and Nitrate.

This is the basis of the aquarium Nitrogen Cycle. However they can be briefly overwhelmed if there’s suddenly too much of either substance to feed on. Also, if your Nitrifying bacteria have been killed off for some reason you may see a spike in Ammonia that can even lead to fish deaths.

Most aquarists only keep pH and Ammonia tests kits. However I recommend keeping an Aquarium Master Test Kit on hand. You may not always need to test for Nitrate and other parameters. But eventually you will and buying each kit individually costs significantly more over the long run.

Also, if you don’t have a test kit readily on hand most pet stores will perform the tests if you bring in a fresh water sample. Make sure it’s no more than a couple of hours old for an accurate reading. Bacterial action, sunlight, and temperature can all affect water chemistry over time.

Are You Overfeeding?

Overfeeding may be the most common reason your fish tank smells bad. Aquarium fish will gorge themselves because they have wild instincts. In nature they can’t count on food being continually available so they will stuff themselves at every opportunity.

Watch your fish when feeding. When full you’ll often see them instinctually snapping up flakes only to chew them and spit them right back up because there’s no more room for food.

As a rough rule of thumb for small community fish like Bettas and Tetras you can estimate their stomach to be as large as their eye. Only provide as much food to fill that space per feeding but feed 2-3 times per day.

If too much food goes uneaten it begins to rot, fouling water chemistry and creating a distinctive aroma.

How Do I Fix Smelly Aquarium Water?

Here are a few of the most effective ways to fish a foul smelling fish tank:

Look For Signs of Stress

Stress isn’t a cause of smelly aquarium water but rather a symptom of it. Investigate to see how your fish and plants are behaving; it may give you clues to how to proceed.

Signs of stress in fish include:

  • Clamped fins
  • Rapid breathing while inactive
  • Staying near the surface
  • Pale coloration
  • Frayed or bloodshot fins

Signs of Stress in Plants Include:

  • Pale or dead leaves
  • Rotting stems or roots
  • Holes forming in leaves

Any of these can indicate conditions like disease, elevated Ammonia, Nitrite, or Nitrate, or something else that’s related to the foul water aroma.

Massive Water Changes

Water changes are one of the main tools in the aquarist’s arsenal. While often time-consuming and inconvenient there’s no better fix for a smelly tank than filling it with fresh, treated water.

However large water changes should always be done infrequently because changing more than 40% at a time can be stressful itself. If you don’t perfectly match parameters (pH, temperature, salinity, hardness) you’re causing a significant shift in chemistry to happen all at once.

Tank bred aquarium fish are quite flexible when it comes to chemistry. But if they are already weakened by elevated ammonia or a disease a sudden pH swing can wipe out your tank.

Since there’s a good chance your fish are stressed due to a toxic agent in the water I recommend using tap water conditioners like Seachem Prime that contain slime coat stimulants. A healthy slime coat bolsters the disease and chemical resistance of all fish.

Check and Clean Your Filter

Is your filter media clogged with slowly decaying food or a swept up fish carcass? The media may need rinsing or replacement. Healthy filters tend to grow thick colonies of bacteria to break down organic and nitrogenous wastes.

However these bacteria sometimes become so numerous they prevent the free flow of water. That doesn’t mean you should throw out the entire colony, though! That’s one of the main causes for New Tank Syndrome in established aquariums.

A simple rinse and squeeze will help media thick with bacteria flow freely again without risking elevated ammonia, nitrites, or nitrates.

If you don’t have a filter for your fish then it’s about time you considered getting one. Betta and Goldfish keepers that go without filters are going to run into the problem of smelly water constantly because there’s no room for beneficial bacteria to grow.

Even a simple sponge filter will go a long way towards providing extra surface area for bacterial colonies and mechanical filtration for suspended particles.

Chemical Fixes for Smelly Water

In my opinion chemical fixes should always be the last solution for smelly aquarium water. They rarely address the problem at its source and you risk letting the unknown issue fester by simply eliminating the odor.

However these agents are extremely effective and work well in tandem with the above solutions. Seachem in particular makes great products for fine tuning your aquarium’s water chemistry. If your fish tank smells bad Seachem Purigen is a rechargeable resin that chemically binds suspended organic compounds.


There are several reasons why your fish tank might smell bad. Decaying fish or food and stagnant water are the most common reasons. Fortunately they can be quickly and easily treated with a little investigation into their causes!

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

Leave a Comment