The following was a quote found on info regarding Melafix. It is the Freshwater section under the thread about Melafix...
Both Melafix and Pimafix are organic compounds. The bacteria in the tank water (not necessarily the ones on the fish) these products don't kill, sometimes use the Melafix and Pimafix as food! This means that when you add these medications to a tank (especially a tank that has not been maintained well or one that hasn't had regular water changes) there is a small chance that a bacterial bloom will ensue and take up dissolved oxygen. This could mean that you could see, under such circumstances, your fish significantly increase their respiratory rate.