First Aid for a Sick Fish: The First Hours

When a fish looks ill, act in the right order: test the water, do a conservative change, isolate, reduce feeding, observe the symptoms, and only then treat. What not to do.

When a fish suddenly looks ill, the first hours matter, but rushing to medicate usually makes things worse. Many health problems can be traced to bad water conditions, and fish will not recover in poor water; optimum water quality, by contrast, actively aids recovery. So the goal in the first hours is not to diagnose a disease and dose it, but to remove stress and stabilise the environment while you gather information. Work through a calm, fixed sequence before reaching for any treatment, and resist the urge to act on a guess.

The first-aid sequence

Test the water: ammonia, nitrite, nitrate, pH and temperature. Poor water quality is the number-one cause, so this comes before anything else.
Do a conservative water change if parameters are off, using dechlorinated water matched for temperature and chemistry.
Isolate severely ill or harassed fish in a hospital/quarantine tank with stable, clean, well-oxygenated water.
Stop or reduce feeding so uneaten food does not foul the water while the fish is stressed.
Observe and note the specific symptoms (spots, fungus, clamped fins, gasping, bloating, red streaks) to identify the problem.
Only then choose a single, targeted treatment based on what you have identified.

What not to do

Do not dump in random or multiple medications; some are toxic to certain fish and to invertebrates, and combining them can make things worse.
Do not medicate before you have identified the problem.
Do not crash-change everything at once with mismatched water, which adds shock on top of the existing stress.

When it is a true emergency

Some signs demand immediate action rather than watchful waiting. Fish gasping at the surface, lying on the bottom, or a sudden multi-fish decline point to an oxygen or poisoning problem: increase aeration and surface agitation, and do an immediate water change with properly conditioned, temperature-matched water. A suspected ammonia or nitrite spike, or accidental contamination such as aerosols or untreated tap water, is treated the same way, by diluting the water fast and restoring oxygen. In these cases a large change is justified, but the replacement water must still be dechlorinated and close to the tank's temperature and chemistry, because a cooler or chemically different change can itself stress the fish and trigger problems such as whitespot later on.