This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Most important: watch your fish and know what their normal behavior
and appearance is. If you don't know what normal is, you can't know
what sick is.
* Clamped fins (fins are held abnormally close to body)
* The fish refuses its usual food for more than 2 days.
* There are visible spots, lesions, or white patches on the fish.
* The fish gasps at the surface of the water.
* The fish floats, sinks, whirls, or swims sideways.
* The fish shimmies (moves from side to side without going forward).
* A normally active fish is still.
* A normally still fish is very active.
* The fish suddenly bloats up, and it's not due to eggs or young.
* The fish is scratching against tank decorations.
I suggest setting up a fish medicine cabinet. It seems like fish
always get sick when the store is closed.
* Water quality test kits: pH, ammonia, nitrite, nitrate
* Aquarium salt (NOT table salt. Most table salts contain additives
to keep them from clumping. Kosher or rock salt is OK).
* Malachite green/formalin ich remedy
* Methylene blue
* Chlorine bleach for disinfection
* Maybe one antibiotic (Kaynamycin or Furanace)
* Antibiotic-containing food
* Copper remedy for parasites
And for fish big enough to handle:
* Malachite green or mercurochrome