Fish Symptom Finder

Last updated: November 2026

Tick every sign you can see, add a little tank context, and get ranked likely causes with what to do in the next hour. Everything runs in your browser — nothing is sent anywhere.

Act now

If fish are gasping at the surface or several fish are dying, don't wait on a tool. Do a 40–50% water change with temperature-matched, dechlorinated water right now, then keep reading. Severe cases: consult an aquatic-experienced vet where available.

1. What do you see?
2. Tank context
Who's affected?

Related first-aid guides

Lethargic & not eating

Bottom-sitting, food-refusing fish: the 5-minute water triage.

Fish dying in a new tank

New tank syndrome explained, with the day 0–42 timeline.

Gasping at the surface

The emergency protocol when minutes matter.

Free: New Tank Cycle Checklist (PDF)

Most of the causes this tool finds trace back to an uncycled or neglected tank. The checklist prevents that.


Or download the checklist PDF now — no email needed.

How this tool works

Each candidate cause carries a weight for every symptom it explains, plus context adjustments — a tank under about five weeks old boosts ammonia and nitrite poisoning heavily, several affected fish boosts contagious disease and shared water-quality problems, a nano tank boosts oxygen and temperature instability. Causes are ranked by combined score. It's an educated triage helper built from standard freshwater husbandry, not a diagnosis.

Severe cases: consult an aquatic-experienced vet where available.