fin rot or other fish biting him?

[britth]

i have 2 goldfish one is small one is big, the big one seems to be lossing his tail. i dont know if the smaller fish is nibbling on his tail or if its fin rot, every few days the goldfish's tail will look more chewed up. but i never see the small goldfish nibble on him

 

[maryl1]

Try moving him to a different tank where he is alone for a while to see if he improves. Some fish are more aggressive even if they are small.
My mother had a oscar once and it was a huge fish, It started to rot from the outside, it had huge holes in it body. I do not know what causes this or if there is anything you can do. I would try to move him to a different tank anyway because if he is sick you don't want the other fish to catch it.