Tank: 20g Fish: 1 Koi/goldfish hybrid(sick fish), 1 goldfish, 3 corydoras, 1 pleco Number of cleans: 1 per week, about 10-20% of the tank. Number of sick fish: 1 Symptoms: loss of colour, two small red patches, loss of appetite, sunken eyes, 2-3 white patches(not ich - they look almost like moss or mold) less then 48 hours since symptoms appeared Hi Everyone! My koi/goldfish hybrid(was bred in the pond years ago, still in the aquarium since he was a baby) looks really, really sick. I've never had any issues with this fish whatsoever, and after doing some extensive googling(turns out sunken eyes are a pretty uncommon symptom), i found Goldfish Ulcers and it appears like that it his condition. I've inspected him for flukes and it doesn't appear like he has any, his gills seem to be uneffected other then the drain of colour on him in general. I've read this is how to cure Ulcer's: Change the water. Your fish won’t recover in less than perfect conditions. Scrub the ulcer with hydrogen peroxide on a cotton ball. Salt the tank with 3 teaspoons to the gallon. Here's the problem, this is a 30gallon tank with 6 fish in it. My "isolation" tanks don't have heaters, and the last time i put a fish into it died, so i'm a little concerned about that. Will continuing the tank cleans be enough? Will Hydrogen peroxide kill the other fish? I'm assuming aquarium salt is safe as long as it's dissolved. Thanks in advance!