May we count your visit with Google Analytics? It uses cookies and sends visit data to Google. You can change this any time under Cookie choices. Privacy

aquarium.guide
Choose your tank

Fish pages, lists, fit marks and tools use the tank you pick here.

Plan everything for this tank →
Menu
Aquarium guides/Why does my fish tank smell? Causes and fixes

Why does my fish tank smell? Causes and fixes

A healthy aquarium smells faintly earthy, like a pond. A strong bad smell means something is rotting: uneaten food, a dead fish or snail hidden in the decor, decaying plants, or a filter clogged with sludge. Find and remove the source, vacuum the gravel, rinse the filter media in tank water, change water and test ammonia.

Cleaning inside a tank by hand
Shown: cleaning inside a tank by hand. Photo: U.S. Fish and Wildlife Service, Public domain, via Wikimedia Commons

On this page

Open the equipment planner →
On this page

What the smell tells you

  • Rotten or fishy: decaying food or a dead animal. Count your fish and look behind decor and in the filter.
  • Rotten eggs: hydrogen sulfide, a gas that is highly toxic to fish, from pockets of deep sand or debris with no oxygen; disturbing a pocket releases it. Do not dig up deep sand with fish in the tank: siphon debris off the surface during water changes, keep the surface moving, and watch for fish gasping at the surface. Gasping at the surface.
  • Swampy or musty: a filter clogged with sludge, or rotting plant leaves.

What to do

  1. Remove the source: dead animals, rotting leaves, uneaten food.
  2. Vacuum the gravel and change 25 to 30 percent of the water.
  3. Rinse the filter media in old tank water, never under the tap. Filter maintenance.
  4. Test ammonia and nitrite; rotting waste raises them. What your readings mean.
  5. Feed less from now on. How often to feed.

A tank that smells even after a clean often has too many fish for its filter. Check the stocking.

Next checks