The short answer
A bad smell almost always means something is decaying — leftover food, fish waste, a dead fish or snail, dying plants, or gunk trapped in a neglected filter. A healthy tank smells faintly earthy and fresh. A strong rotten, sulphurous or musty odour is your tank telling you that organic waste is breaking down faster than it’s being removed. The fix is to find the source and clean it out.
Common causes
- Overfeeding — uneaten food rots on the substrate. This is the most frequent culprit and also drives ammonia spikes. See our feeding guides.
- Decaying matter — a dead fish, snail or rotting leaves hidden behind decor or under gravel.
- A dirty filter — trapped sludge goes anaerobic and smells foul when disturbed.
- Detritus in the substrate — waste that’s never vacuumed out.
- Anaerobic pockets — deep, compacted substrate can release rotten-egg (sulphur) smells.
Tip: rinse filter media in old tank water, never hot or chlorinated tap water. Tap water kills the beneficial bacteria and can trigger a mini-cycle — see why tap water needs treating.
How to fix it
- Do a water change and vacuum the substrate with a gravel cleaner to lift out trapped waste — follow how to do a water change.
- Remove any dead matter and trim dying plant leaves.
- Clean the filter gently in tank water if it’s clogged; browse filter guides if yours is undersized.
- Feed less so waste stops accumulating.
Keep it fresh
Regular maintenance is what keeps odour away for good — consistent water changes, sensible feeding and an occasional filter rinse. See our maintenance hub, and if the smell comes with cloudy or discoloured water, check why aquarium water goes cloudy.