I think the only way a prophylactic treatment can fail is if you're treating for something the fish doesn't have, or the fish is too far along (or you're not treating properly). Personally, I use Cupramine, Prazi, and Kanaplex for fish. Metro is more for coral health. My way treats Ich, Velvet, Flukes and internal bacterial stuff. I have NEVER had a fish die in QT, or had a sick fish in my display. One thing I will say is that I've had some fish species go blind after roughly 2 years. I don't know if that has to do with the treatment, but it seems like a while for the fish to go after QT and then go blind. Also, I QT 1 fish at a time in a 12 gallon Fluval Evo tank so it's easy, and inexpensive, to do complete water changes between meds. Plus I use less meds in a smaller tank. Every fish has been very happy and energetic in QT during treatment. Probably because they see the bigger tank and want to get to it, lol. I find putting a piece of cardboard that blocks 1/3 of the viewing area works better when the fish wants to disappear than pvc pipe that they never use. Then, when they want interaction or food, they swim where they can be seen. Stresses them less.