he (or she) is about 9 years old. he's about 9 inches long. he's always been the only fish in the tank. we initially bought him along with several other fish. he was about the size of a quarter. we didn't know his nature. he killed all the other fish and has been alone in the tank ever since. i would like to add an algae eater to help keep the tank clean. are there any algae eaters that he won't kill? if yes, will it need to be of a minimum size?