A long time ago (and I mean a long, long time ago) I gave both Pimafix and Melafix a shot. I think at the time it was being touted as the latest, greatest thing. I used it for a couple of weeks, saw no improvement, so I went back to my old tried and true: Maracyn 1 & 2. Unfortunately, it was too late and the fish died. So, I wrote it off as snake oil and never used it again. Now, to be fair, this was a long time ago so perhaps they've improved their formula between now and then. But I think the last time I saw Pimafix/Melafix in a LFS it was still touting "extract from the Tea Tree" as it's main ingredient. I'm sorry but that's just BS. Maybe it can help bolster a fish's natural immune system but it's not a real medicine IMO.
Btw, it was that experience with Pimafix/Melafix that made me realize that not even Maracyn 1 & 2 is very effective at treating something that's at an advanced stage. Early detection is the key. I've got some Furan-2 on hand and plan on giving that another go the next time my LFS calls me with a really bad bacterial disease.