When they do get sick, though, it always helps to notice their condition early.

If any of these characteristics look suspicious, test the water to make sure there are no problems there. As a precaution, clean the filter and change a third of the water in the tank. When you suspect that any of the fish may be sick, do not add any medications to the tank.

You should not treat the tank without knowing what the fish is suffering from. Many fish medications are ineffective or contain so little medication that they do no good at all. Antibiotics in particular are a problem. If used in small amounts, the bacteria they are supposed to kill can develop resistance to the drugs.
When used at much higher doses, the nitrifying bacteria in the biological filter can be wiped out. Most medications include instructions to change some of the water before each dose.