Looks like a young male and could be getting bullied by a bigger male. It also has a really fat abdomen, which could be worms, a growth of some sort, or it just eats a lot.
Make sure the water quality is good (no ammonia, no nitrite and less than 20ppm nitrate, pH around 7.0 or slightly above).
Make sure the fish is eating well.
Monitor the fish's poop. If it does stringy white poop it has an internal problem.
Make sure you have some floating plants to offer shelter and hiding places.