Maracyn or any other antibiotic does exactly what it implies kills bacteria. And it will kill your beneficial bacteria just as it kills the harmful bacteria. Using maracyn in a main tank will cause you to have to recycle your tank. i made the mistake a while ago and lost several expensive payara fish. Around 600$ dollars worth to be exact because after the parasites were treated the tank started to cycle unknown to me and a ammonia spike killed everything in it.--------------It is highly recommended to take the sick fish out of the main tank and put them in a hospital tank to treat them with this antibiotic.