I think that's a large part of why people prefer hospital/quarantine tanks to treat only the sick fish. Also, it is why many people try to avoid medications and look for more natural remedies (clean tank, well-varied diet, garlic, StressCoat+, etc.)
The reason I was wondering is, I have a little oranda baby with cotton mouth (came to me that way) she is in my 10gal Q tank and is being treated with API pimafix, it is day 4 of the 7 day treatment and the cottony growths are just about gone, but I was worried about my good bacteria, it took forever to cycle that little tank lol..thanks for the replies.
you'll prbably have to build the cycle back up in your q tank after you are done with the treatment. I, along with some others here, have tossed around the idea of using Nurtrafin Cycle in a hosp tank that is set up temporarily, then you don't have to worry about keeping an additionsal filter cycled.