The short answer
A sick fish usually shows a change from its normal self — and the general warning signs are worth knowing. The most common are clamped fins, spots or patches, lethargy, unusual breathing, and not eating. If you notice these, your first move is always the same: check your water quality, because poor water causes or worsens most illness. For anything beyond the basics, seek help from a vet or experienced fishkeeper — don’t try to self-diagnose a specific disease.
Common general signs to watch for
- Clamped fins held tight against the body instead of spread.
- Spots, patches, film or fuzz on the skin, fins or gills.
- Lethargy — resting on the bottom, hanging near the surface, or hiding when it normally doesn’t.
- Rapid or laboured breathing, or gasping at the surface.
- Loss of appetite or ignoring food it usually loves.
- Fading colour, or unusual swimming (rolling, darting, scraping against objects).
None of these points to one specific disease — they’re your cue to investigate, not to reach for a bottle of medication blindly.
What to do (and not do)
- Test the water and do a water change if ammonia or nitrite is above zero, or nitrate is high.
- Check the basics: temperature, filtration, recent changes or new additions.
- Isolate a clearly unwell fish if you have a spare tank, so it can rest and any problem stays contained.
- Don’t medicate blindly. Random treatments stress fish and can crash your filter bacteria.
- Get expert help for a specific diagnosis — an aquatic vet or a reputable, experienced community.
Prevention beats cure
Most illness traces back to husbandry. Stable water, sensible stocking and a steady routine keep fish resilient. Keep up water testing and aquarium maintenance, quarantine new fish, and address stressors early — see why is my fish hiding? and why is my fish gasping at the surface?. When something serious is beyond your experience, always defer to a professional.