UV is best cure and prevention along with garlic. I understand your concern of the bacteria loss for the bio pellete's .feed em and watch them. Feed heavy with garlic. We just went thru this when mrs Binford got me a lemon peel from LFS . Killed off 17 fish. Tho I think it was velvet gill and ich. Added a larger UV and survivors are looking good again. If You have a fish trap might want to set up a small copper tank and treat the ones you catch.good luck and the nice thing is ich only has a fourty day life cycle from what I have been told.