Fish tank smells: what the smell means and how to find the source

A healthy fish tank smells faintly earthy, like a pond or wet soil. If your fish tank smells of rotten eggs, rotting meat or sewage, something is decaying: usually a dead fish or snail, uneaten food, a clogged filter, or a pocket of gas trapped in deep sand.

A bad smell is a warning worth acting on today. The things that cause it also pollute the water, and the rotten-egg gas, hydrogen sulphide, is highly poisonous to fish. Your nose is a useful tool here, but it can't detect ammonia at levels that harm fish, so test the water too.

How to identify it

Name the smell first. It points to the cause.

Earthy, like soil or a pond: normal. Healthy bacteria, plants and algae produce this.
Rotten eggs or sulphur: hydrogen sulphide gas, made by bacteria working without oxygen. Usual sources are deep sand beds, sand under rocks, a neglected canister filter, or a buried dead animal.
Strong, sickly rotting smell (like old meat): a dead fish, snail or shrimp, often hidden behind rocks, inside decor, under the substrate or inside the filter.
Sour or fishy, with cloudy water: rotting food from overfeeding, often with a bacterial bloom. See cloudy water.
Musty or swampy from the filter: a filter packed with sludge that hasn't been cleaned for months.
Chemical, perfume or soap: something got into the tank. See soap or chemicals in the tank.
Burning plastic or hot electrical smell: unplug the equipment now. A heater or pump may be failing.

When it's a problem

Normal: a mild earthy smell when you lift the lid; a stronger smell from a filter you've just opened for cleaning.

Early: a slightly sour smell or a faint whiff of sulphur near the tank, fish acting normally.

Serious: a smell you notice across the room, cloudy water, or ammonia or nitrite showing on a test.

Emergency: a strong rotten-egg smell with fish gasping, lying on the bottom or dying. Do a large water change now and go to fish gasping for air.

What to do right now

Count your fish, snails and shrimp. A missing animal is the most common cause of a sudden bad smell.
Test the water for ammonia, nitrite and nitrate. A decaying body or food releases ammonia fast.
Sniff each part separately: the water surface, the filter outflow, the filter itself, and the substrate after gently lifting a rock.
Remove anything dead or rotting, including mushy plant leaves and old food.
Do a 30–50% water change with dechlorinated, temperature-matched water if anything tests above zero or the smell is strong.
Add surface agitation with an airstone or by lowering the water level slightly so the filter splashes. Oxygen helps the gas escape and stops more forming.

What to do next

Rotten-egg smell from sand

Sand deeper than about 5 cm (2 inches), and fine sand under heavy rocks, can develop pockets with no oxygen. Bacteria there produce hydrogen sulphide. You may see black patches in the sand or gas bubbles rising when you push a finger in.

Fix it slowly. Don't dig the whole bed up at once: a big release of gas can poison your fish. Instead:

Stir a small section, about a quarter of the tank, each day or two, then do a water change.
Use a gravel vacuum to clean the top layer of sand at every water change.
Thin a very deep bed to 2–5 cm over a few weeks.
Rest rocks on the tank bottom or on a support, not on top of deep sand.
Malaysian trumpet snails burrow through sand and help keep it turned over.

Water changes and activated carbon help remove the smell while you fix the cause.

A dead fish or rotting food

Search thoroughly: behind the background, inside caves and ornaments, under the filter intake, and inside the filter itself. Small fish get sucked into filters. After removing it, test ammonia daily for a few days and change water whenever it reads above zero. Cut back feeding: see overfeeding.

A dirty filter

Rinse sponges and media in a bucket of old tank water, never tap water, which kills the helpful bacteria. Don't clean everything at once; clean the filter and do a gravel vacuum a week apart. Canister filters often need cleaning every 1–3 months. See cleaning your tank.

Cautions

If your fish look sick after a gas or pollution event, don't add medicine right away. Fix the water first. If you do medicate later, remove carbon, and take extra care with plecos, loaches, scaleless catfish, shrimp and snails.

Products that help

Gravel vacuum (siphon): removes rotting waste from the substrate at each water change.
Liquid test kit: shows if the smell has already polluted the water with ammonia or nitrite.
Water conditioner: for the emergency water change.
Activated carbon: removes odours and dissolved organics; replace monthly, and remove it when medicating.
Air pump and airstone: adds oxygen and helps gases escape.
Long tweezers or a net on a pole: for reaching dead animals behind decor.
Burrowing snails: keep sand from compacting.

How to prevent it

Count your fish at every feeding.
Feed only what is eaten in about two minutes.
Vacuum the substrate and change 20–30% of the water weekly.
Clean filter media in old tank water on a schedule.
Keep sand beds shallow (2–5 cm) unless you know how to run a deep bed.
Remove dying plant leaves.
Keep the surface moving.

Why it happened

Bad smells come from decay. When rotting material builds up faster than your filter and water changes can remove it, or ends up somewhere with no oxygen, bacteria break it down into foul-smelling and often toxic gases. Overfeeding, a hidden death, too-deep sand and a neglected filter are the usual reasons.

When to get help

Get help if the smell keeps coming back after a thorough clean, if fish are dying, or if you can't find the source. A local fish shop can test your water. If the smell is electrical, unplug everything and replace the faulty equipment.

In the game

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