Mine used to see his own reflection at the other end of the tank and he would freak out. He would fluff himself up, they have this neck thing (I don't know the technical term) that poofs out --kinda like the dinosaur inJurassic Park.
He would swim really fast to the end of the tank where he thought the other fish was, bump his head on the glass, and start all over!!!
Marie-Eve and Buster (5 year old-ish rescued Boxer)
Deep thought, by Jack Handey : "I think my new thing will be to try to be a real happy guy. I'll just walk around being real happy until some jerk says something stupid to me."