so funny, that fish is the only fish in the tank? way to funny. the mean bugger !! my female fire mouth was like this, she was a bigger. but she finally got used to the big tank and the others. but the other 3 fire mouths OYe there driving me bonkers.
1 can live their whole life in a 55g, but its best to keep then in shoals of at least 4, mine is antisocial and doesn't like other piranhas, so he'll probably be fine by himself
these are the most boring fish ive ever owned! lol they don't swim! haha and don't trust them with plecos, Ive seen some REALLY beat up ones from being in piranha tanks, you could try it but don't feel bad when it disappears.
People say they do best in groups, but Ive seen them by themselves and their fine. Its not like they swim around with each other, they just sit there so I don't really understand why people think they should be in groups. If you keep him by himself in that 55 gallon he will probably get a really healthy size.