The active swimming will likely slow down as the fish matures... It will probably be able to squeak by in a 55g tank, but will take a bit more coordination to turn around once it hits its full size. (it will probably take a few years to hit full size anyway, and it'll be fun to watch it grow up) For any newbies checking out this thread, I wouldn't recommend trying this, as it would be "ideal" to have 5+ in a 125+ gallon tank, but there are always exceptions to the rule. Nicely done somethingpishy1, looks like your piranha is healthy and happy. 
