If you had a severe outbreak in a tank and want to basically sterilize it after removing all your fish from it, hydrogen peroxide can kill basically any potentially harmful microorganism, even more effective than methylene blue, but cannot be used safely with fish the same way methylene blue can. I would only use hydrogen peroxide after an outbreak killed off fish if I needed to completely reset the biology of a tank. #fish #goldfish #aquarium #ranchu | Luke’s Goldies