they're just carnivores, it's just how the food chain works
there are four species of po fish: the pomp, pompy, pompano and nibbono species. each live on their own planet within the solar system, with the order from the sun being pomp, nibbono, pompano, pompy respectively.
pomps are the basic and most numerous species, they are stupid, weak and slow
pompies are even stupider than pomps, but have an extremely hard shell which makes them unbelievably durable
pompanoes are the smartest of the four species and have hard shells, though not as tough as pompies. they live in a technologically advanced society and are pathological clean freaks
nibbonos are outsiders; they arrived to this half of the solar system later than the rest, although still many billions of years ago, and they are about as smart and tough as pompanoes, but like filth instead of cleanliness. they are carnivores and used to eat pomps before the four species collectively stopped needing to eat. the ones that still eat pomps are the reputation ruiners (who i posted a while back)