i have just noticed that my goldfish (which is white with a really long tail) has a reddish mark on his tail. it was never there before and it looks like it might be swollen. also there is very little thin pieces of white coming from it. it looks like the tail could maybe be split a little and bruised. but i'm not sure. i have other goldfish in the tank with him and a little cold water algae eater, could one of the others have done this to him? does anyone know what it could be and is it serious?