My 6 yr old son is REALLY into fish right now..mainly sharks. He wants a tank in his room but the largest it could be is 30g. Ou r 55 houses discus. He has only said he wants a shark or a piranha, but neither will fit in a smaller tank. I tried to show him red finned sharks but he didnt buy it. Are there any smaller fish that he would think are cool? I really want to encourage his interest- good hobby, and he says he wants to be a marine biologist. He even got a piranha book at the lfs and has me read it to him!