Have you considered that your problem may be overtreating? Ever heard the saying "snort, sniffle, sneeze, no antibiotics please!" Basically, what that means is that if you fish are showing signs of problems, but the problem isn't bacterial, adding antibiotics to your tank may actually harm them. The problem could be viral or parasitic, but by adding antibiotics you're actually lowering your natural "good" bacteria levels, thereby messing up your tanks natural cycling due to unnatural bacterial fluxes. Sometimes the best remedy is just to leave it alone. No water changes, no antibiotics, no new fish. Let your tank cycle completely on its own, and if after 6-8 weeks everything is doing fine, then try adding new fish. And add them slowly! No more than a few inches a week (you are following the 1 in. per gallon rule, right?). I've found that the healthiest tanks I've ever had were the ones I never messed with. Let them be their own ecosystem.
Tara