This just started happening about ten minutes ago. My goldfish, one Oranda and one classic goldfish, have been nudging each other and kind of chasing each other. I'm not sure if they don't have enough space, are trying to show dominance, are trying to mate, or if they're just playing. I don't know the genders of the fish, but as far as I know they wouldn't mate because they are different breeds. I'm sorry if I'm wrong about that, please correct me if I am. I'm pretty sure they aren't fighting for food. I fed them this morning, and they both got their fair share of food. The video above shows what they've been doing.