I don't know dude melafix is supposed to be safe for the fish. Maybe try placing all your infected fish in a temporary holding tank I know it brings a huge amount of stress to the fish but than again not so much... For instance this one time I accidently killed off all of my natural bio-filtering bacteria and therefore caused a ammonia epidemic which sent all of my bottom feeders jumping and all they could to get a gasp of air... And anyways one of them (a 14 inch channel catfish) got seriously pale white and stopped breathing... I thought he was dead but decided what the hell and threw him into a cooler with clean dechlorinated water and walah a miracle the fish started breathing and regained his color and still he sits in my tank a very happy healthy fish! But I digress I'd recommend placing them outside of the tank.. and or using a diluted amount of chemical.. and definately do a water cycle that will help all the fish maybe a 10 percent and try to get all the gunk out of the ground with a gravel pump. hope that helps I still am kind of a noob at fish keeping but I have had several occassions like yours where and ich outbreak kills all my untainted clean healthy fish and leaves the other sick infested ones alive.