when i had a problem like this with my clowns i fresh water dipped them, waited one day and did it again and it never came back, oh while i was dipping them i let the clear container float in the tank so they could see their home, less stressful for them than sitting somewhere they have never seen. it works. if you decide to try it read up on it. and you have to catch it early when the fish is still eating and not to far gone. i also have a uv sterilizer which i feel helps with things like this. i dipped the fish and back into the water they went.