ive had a bad experience with melafix. ive also read many other peopel on here have had bad experiences with it. id ditch it and get a real medicine like maracyn. also it doesnt really matter a whole lot cause i think if you leave your puffer in the same tank with the clown you will eventually not have to medicate the tank cause your clown will be dead. what kind of clown and puffer? maybe if you have a maroon clown and a small puffer but other than that i wouldnt think it would work and apparently its not. you should really take this as a hint you need to do something with one of your fish. hope this helped.