A healthy aquarium smells like almost nothing — a faint earthy note at most. If your tank smells bad even after cleaning, the odour source is something cleaning doesn't reach. Work through these six causes in order; one of them is your answer.
1. Something died where you can't see it
The #1 cause. A fish or snail decaying behind hardscape produces powerful rot smells within a day. Do a headcount, check behind rocks, inside decorations and under driftwood. Snails smell worst — an empty shell with a foul odour is your culprit.
2. Overfeeding (the slow-burn version)
Food that disappears from view isn't gone — it rots in the substrate and filter. If you feed more than the tank finishes in a few minutes, this is at least part of your smell. Portions and schedule: our feeding guide. A clean-up crew of snails and shrimp processes leftovers before they can rot.
3. Anaerobic substrate (rotten egg smell)
A sulphur/rotten-egg smell when you disturb the gravel = pockets of oxygen-free decay in a deep or compacted substrate. Fix: vacuum a different section weekly (never all at once), keep sand beds shallow, or let Malaysian trumpet snails burrow and aerate it for you — they're the natural cure.
4. A filter that's become the problem
Filters collect everything; if the media never gets rinsed, the filter itself turns into a sewage box. Rinse mechanical media in removed tank water (never tap — chlorine kills your bacteria) every few weeks. Smell strongest right at the filter outflow? This is your cause.
5. Water gone stale
Musty, swampy smell + it's been weeks since a water change = accumulated organics. 10–15% weekly, temperature-matched. If numbers have drifted too, run the checks in our water quality guide.
6. Surface scum blocking gas exchange
An oily film on the surface traps gases in the water. Skim it with a paper towel and add surface agitation (airstone or angled filter outflow) — smell often improves overnight.
Prevention: the no-smell routine
- Feed only what's eaten in minutes · weekly partial water changes · vacuum in rotation
- Rinse filter media monthly in tank water
- Keep live plants — they consume the nitrogen compounds that fuel odour
- Run a clean-up crew — nerites, ramshorns and shrimp are a 24/7 sanitation team
FAQ
My water is crystal clear but stinks — how?
Clear ≠ clean: dissolved organics are invisible. It's usually cause 2, 4 or 5 — and if clarity IS also off, see crystal clear water guide.
Is the smell dangerous to my fish?
The smell itself isn't, but its source (ammonia, hydrogen sulphide) can be. Treat a sudden strong odour as an early-warning alarm — test the water the same day.
Build your sanitation crew
The cheapest permanent fix is biological: snails, shrimp and algae eaters from our Sydney tanks, shipped with a Live Arrival Guarantee.
