Why is my fish eating all my other fish? (continued)

It;s definitely a black moor, and it's with other goldfish that are bigger than him.

Then I would say he is just a very aggressive Black Moor and he should be kept separate from other fish.

The species is known for being peaceful but it doesn't always mean the fish will follow the stereotype. I've seen betas be peaceful in a community tank before(and they are commonly known as a very aggressive fish.). You just have to test out your fish to see if they are compatible and if they show aggressive behavior to each other then you have to separate them.

flush it!

flush it!

A: Your fish is evil
B: He was raised by wolves
C: He just wants to be alone

The answer is C. Actually moors really are very territorial and are normally agressive to other fish. A bigger tank might help but he would probably be happier in his own bowl.