My mum gave me a tiny comet goldfish a couple of years ago & it's grown massive. This morning I noticed a large white mark on his head, I have no idea what it is & it wasn't there yesterday. Could it be an injury from trying to jump out of the tank? I did a partial water change yesterday but I always use the same chlorine neutraliser & there's been no change to his routine or anything. I've googled it & read some books on goldfish care but it doesn't look like any of the diseases in them. Does anyone know what this might be?