Depends on what's in your tank and whether any other fish are at risk for developing the same thing. you don't want to medicate healthy fish. Do you have a large-ish tupperware sort of container? Stick a cheap filter in it and presto - you have a hospital tank.
Also, sometimes fish can get sick for no apparent reason, but other times we can figure out whether something in particular caused his illness. If you can fill out the questions below, we'll take a stab at trying to figure this out.
Tank size:
pH:
ammonia:
nitrite:
nitrate:
tank temp:
Fish Symptoms (include full description including lesion, color, location, fish behavior):
Volume and Frequency of water changes:
Chemical Additives or Media in your tank:
Tank inhabitants:
Recent additions to your tank (living or decoration):
Exposure to chemicals: