What kind of mark is it?
Melafix is a bacterial treatment for things such as fin/tail rot, red sores, mouth fungus, pop eye etc. If its one of these then use melafix.
Pimafix however is a fungus treatment and is used for things like white fluffy cottom like growths or deposits on the fishes scales. You need to assertain what your fish is suffering from.
Incidently what tank size do you have, what ph are you running and what are your amonia, nitrate and nitrite levels? Before you start a course of either melafix, pimafix or for some people both, do a 50% water change, then treat every day for 7 days. After this perform another water change and repeat medication if needed.
If you have a large community of fish, and the incidence is isolated then its best to quarantine the fish in a separate medication tank, and treat the fish, rather than treating the tank and risk overmedicating the healthy fish.
