New Catfish\Shark

[SiestaSkyy]

Recently I was out at a couple fish stores with a friend. I needed something else to help my tsn clean up the leftovers off the bottom of my tank(all babys) so I bought a couple random catfish on a whim. According to the store its a "Highfin bull shark" but I did a little research and found it to be a "Black Fin Catfish" (Hexanematichthys seemanni). I love them. They never hide, eat everything that makes it to the bottom, dont get to big and look pretty neat. Here's my problem. They turn brackish to salt as they mature. My TSN will eventually out grow my tank. I knew that going in. What can I get that is comparable to these awesome little guys that I can keep forever?

[SiestaSkyy]

BTW, its a 225gal im looking for something about 12"-20"