Goldfish Problem?

Hi I checked on Ariel my Red Comet Goldfish and I noticed half of her tail fell off. She was very hyper yesterday and she still had her tail. and also my other Goldfish was hyper smashing into the Gl,Diving and banging himself against the bottom of the tank. He doesn't stop so I have to put a net around him so that he will calm down. Could he be territorial and maybe tore off Ariels tail?