Melafix 
Great tonic for use whenever fish show signs of fin rot or other bacterial problems such as ulcers and damaged fins.  Read more about Melafix HERE.
NEW LOWER PRICE!
Melafix16 -- $16.95 16 oz. Bottle (treats 4800 gals)
Melafix64 -- $52.95  64 oz. Bottle (treats 19,000)