How big is the tank for goldfish? Goldfish require a ridiculous amount of tank space, COLD water (rosy barbs need HEATED water).. They should not be housed together again.
I'm not sure for goldfish (SASAMI may be able to clarify this) but you maybe able to use a bit of
aquarium salt (NOT TABLE SALT bc it contains ingredients which prevent caking). And just keep the goldfish in super clean water.
It's been a thousand years since I've owned goldfish but for my betta who nipped his tail I used salt for 10 days and then kept his water clean and it grew back. Do goldfish eat foods other than flakes/pellets? If they are omnivorous-carnivorous try frozen bloodworms because they are high in protein.
***Disregard my response if Sasami comes with a different idea. SHE is the fish expert!!
http://www.goldfish-emergency.com/viewpage.php?page_id=17