mouth injury

My goldfish swims around everywhere with his mouth wide open. It started happening about a day ago. He can still eats and swims fine, but sometimes he isolates himself from the other goldfish in the tank. I attached a photo of what he looks like and there seems to be some kind of substance in...

Hi, My fish pond was recently attacked by something that killed one of my oldest Comets by biting only the mouth off and the second that I thought was dead was still alive. His mouth was injured as well and I was certain he would die but he survived and is swimming around with an injured fin...