If you really want one dude, i was considering it before i turned my 55 into a reef. I'll try to find all of my old links for you. I was going to get an "elongatus" pirahna, they are ugly but everyone says that they are the meanest. But its hit or miss with pirahnas as to which ones are mean, some are passive, and ive seen some attack the glass after people. (true story.) difference is in the single fish not the species. However, all the ones that swim in schools (normal red bellies.) are usually very passive if kept singly, its the ones that live solitary in the wild that are terrors (blacks, elongatus) anyways ill look those links up for you.