I've never used pimafix, but Melafix is 99% distilled water and 1 % Tea tree oil (meluca oil)... so yeah, Melafix is just distilled water and tea tree oil. I have always looked at it like a supplement as well. I don't look at it as the cure per se, but helps with aiding healing of skin related wounds. I use it occasionally in conjunction with water changes. Water changes and good clean water parameters usually are one of the biggest things to help a sick fish recover, or even just preventing alot of problems period. Clean water will help keep your fish healthy and keep their immune system up to par, and also will help to reduce chances of bacterial/fungal infections. It's super basic common sense, but sometimes people overlook/fail to keep pristine water conditions/parameters. Overstocking/overcrowding is another common mistake people make. This also adds stress to the fish(which can lower their immunity and some people even say can bring on outbreaks of ich), a bigger bioload(this has the obvious problem of more waste introduced into the environment which increases the chance for bad water parameters and things like ammonia spikes to happen.)
I know i'm soapboxing and constantly repeating the point of water changes/good clean water parameters, but it's because it is SO important and such an easy fix for preventing problems in the first place and also helping fish recover when a probelm does happen.