That is, I think the problem is parasites. The fish range in size from about 1.5 inches to 6 inches (they get along just fine) and have been scratching themselves on rocks and other tank ornaments. I started using a malachite green/formalin combination which turns the water blue and actually looks kinda cool. Anyway, I've been using 1 drop per gallon of water each night (light tends to neutralize this medication so I dispense it after the light is off the the evening) for about a week now, but they're still scratching. It should be noted that I did not start the medication when I first noticed the symptoms, so there is the possibility that the parasites, if that's in fact what it is, have multiplied or dug their heels in, so to speak, before I started. Otherwise, the fish seem healthy. They're hungry as ever, active as ever, and none of them show any signs of illness other than the scratching.
So, back to my original question: How long should it take this medication combo to completely kill off the parasites? Thanks!
[NOTE: You'll notice that I didn't include details like the number of fish in the tank, the size of the tank, exactly how long I waited before dispensing the medication, etc., because I don't want a bunch of holier-than-thou fish geeks telling me that there are too many fish in the tank, that I waited too long, the tank isn't large enough, etc., in other words, criticisms that in no way shape or form will answer the question and really have no purpose other than showing off how much they think they know].