Likely cause
Bad-Smelling Water
A bad smell almost always means decaying organic matter — uneaten food, dead plant material, or something (or someone) hidden in the decor — or a bacterial imbalance. Do a partial water change, check for anything decaying, and test your water to see if ammonia is elevated.
Why this happens
- Decaying organic matter — uneaten food, dead plant leaves, or an unnoticed dead fish hidden in decor.
- A bacterial imbalance, often linked to an ammonia spike.
- An overdue filter clean allowing trapped waste to break down inside the media.
What to do next
- Check thoroughly for anything decaying, including under and behind decor.
- Do a partial water change and test ammonia and nitrite.
- Clean (don't fully replace) filter media in tank water, not tap water, to avoid killing beneficial bacteria.
- Address the source rather than masking the smell — it won't resolve until the decaying matter or imbalance is fixed.
Frequently asked questions
Why does my fish tank smell bad?
A bad smell almost always means decaying organic matter or a bacterial/ammonia imbalance — check for anything decaying in the decor, then test your water and do a partial water change.