So he is in a 50 Gallon? Yes he has space to move but not to "freely swim". See how his tail fin is arched down? That can be from two things, first is genetics, but second is cramped living space which this guy is living in. If you could put him in a 90 or larger that would help him a lot, plus as convict360 has said since he is in a tank with no gravel, tank mates, decorations, or fake plants to mess around with he is probably just really really bored. If you can't afford to get him a bigger tank then at least give him some gravel he can move around, it won't crowd his space but it will give him something to actively do.