Troubleshooting · Fishkeeping
Something is wrong with the tank: a diagnostic order
Nearly every problem in a freshwater tank is a water problem wearing a disguise. Fish gasping, hiding, clamping their fins or dying one at a time are symptoms with a dozen possible causes, and the diagnostic order is what stops you treating a disease that is not there. Test before you medicate — the commonest harm done to a struggling tank is a keeper adding something.
Work down this list rather than jumping to the symptom that looks most alarming. Each step is cheap, and each one rules out a cause that would make the next step meaningless.
- 1. Test ammonia, nitrite and nitrate — Before anything else, and with a liquid test kit rather than strips. Any detectable ammonia or nitrite is the problem, whatever else is happening — the biological filter is not keeping up, and every other symptom is downstream of it. The immediate action is a large water change, not a treatment.
- 2. Check temperature and whether it has moved — A failed or stuck heater is a common and fast killer, and a sudden swing stresses fish more than a steady wrong number. Check against a separate thermometer rather than the heater dial.
- 3. Look at what changed in the last fortnight — New fish, new plants, a filter clean, a different food, a medication, a new decoration. A tank that was stable and is not usually had something added. Filter media rinsed under tap water is a classic — chlorine kills the bacteria the tank depends on.
- 4. Check flow and surface movement — Fish gasping at the surface most often means oxygen rather than disease, and oxygen is mostly a function of surface agitation and temperature. Warm water holds less. Increasing surface movement is free and immediate.
- 5. Only now consider disease — If the water is right, the temperature is stable, nothing changed and oxygenation is adequate, then look at the fish themselves — spots, growths, fin damage, behaviour. Identify before treating: medicating a tank for the wrong thing costs you the biological filter and makes everything worse.
- Algae is not on this list — Deliberately. Algae is a symptom of light, nutrients and plant mass being out of balance, and it is almost never an emergency. Fix the tank; the algae follows.
Two situations are worth naming because they reward asking rather than experimenting. A tank losing fish steadily despite good water parameters is a diagnosis this page cannot make, and a local aquatic shop or a species society will get further with photographs and a history than any general guide. And any treatment involving copper, antibiotics or a quarantine decision is one to research specifically for the species you keep — invertebrates in particular are killed by treatments that are routine for fish.
This guide is a diagnostic order for a keeper looking at their own tank. It is not veterinary advice and it does not identify diseases.
Questions this guide answers
Listed so you can tell at a glance whether this is the page you wanted. HobbyHQ keeps one guide per question family rather than a page per phrasing.
- why are my fish dying
- fish gasping at the surface
- cloudy aquarium water
- how do I fix ammonia in my tank
- fish keep getting sick
- aquarium algae problem
Part of Fishkeeping. Last reviewed 2026-08-15.