Guide
Top 10 fish illnesses you may see — and what you can do about each one
A practical rundown of the most common problems in freshwater tanks: what to look for and how to respond.
1. Ich (white spot)
What you see: Small white spots like salt on body and fins; flashing, clamped fins, heavy breathing. What to do: Raise temp if safe for your fish; use an ich medication for the full course; keep water pristine. See our full ich guide.
2. Fin rot
What you see: Fins look ragged, receding, or red-streaked; can expose fin rays. What to do: Improve water quality first (water changes, test for ammonia/nitrite). If it’s bacterial, treat with a bacterial medication; reduce stress and ensure no nipping tank mates.
3. Velvet
What you see: Dusty gold or rust film on the fish; flashing, lethargy, fast breathing. What to do: Similar to ich — dim lights, medication (e.g. copper-based or other velvet treatment per label), excellent water quality. Quarantine new fish to reduce risk.
4. Swim bladder disease
What you see: Fish floats, sinks, or lists; can’t stay level. What to do: Often diet or constipation. Try a fast for 24–48 hours, then a small piece of cooked pea (shell removed). If no improvement, consider bacterial infection or internal issue; isolate and treat if appropriate.
5. Dropsy
What you see: Swollen body, scales sticking out (“pinecone”). What to do: Usually organ failure or severe bacterial infection. Isolate; improve water; some keepers try antibacterial food or baths, but prognosis is often poor. Focus on prevention with good water and diet.
6. Fungus (cotton-like growth)
What you see: White or grey fluffy patches on body, fins, or mouth. What to do: Often secondary to injury or poor water. Clean water, remove cause of injury; use a fungal treatment if it spreads. Salt (where safe) can help in early stages.
7. Pop-eye (exophthalmia)
What you see: One or both eyes bulge out. What to do: Can be infection, injury, or water quality. Improve water and reduce stress; if bacterial, antibacterial treatment. Sometimes damage is permanent even if the cause is fixed.
8. Hole-in-the-head / hexamita
What you see: Pits or holes on the head, often in cichlids; sometimes stringy white faeces. What to do: Improve diet and water quality; use a metronidazole-based treatment if hexamita is suspected. Reduce stress and overcrowding.
9. Columnaris (mouth fungus / cotton wool / saddleback)
What you see: White/grey patches around mouth, sometimes on body or as a saddle-shaped band on the back; can spread fast. What to do: Bacterial; isolate and treat with an antibacterial (e.g. for gram-negative bacteria). Improve flow and water quality; reduce stress. See our full columnaris guide.
10. Parasites (internal or external)
What you see: Worms visible in faeces or from body; scratching; weight loss; bloating. What to do: Depends on parasite. Internal worms: deworming food or medication. External: quarantine and appropriate antiparasitic. Always improve water and nutrition to support the fish.
In every case: test your water (ammonia, nitrite, nitrate) and fix water quality first. Log parameters with a tool like App-aquatic so you can spot trends. When in doubt, isolate the fish in a hospital tank and seek advice from a vet or experienced keeper.
Quick takeaways
- Most illnesses improve with excellent water quality and reduced stress.
- Identify the problem (spots, fins, swelling, behaviour) then treat accordingly; quarantine when needed.
- Prevention: cycle the tank, don’t overstock, quarantine new fish, and keep a consistent maintenance schedule.