I'm new to this site, but I was on predatory for a few years. I would keep a fahaka puffer. In fact that's what I'm doing right now. I had a 180 with a bunch cool fish, ....ray, tirinus front, dat,etc....I got the fahaka just keep that fish fix up.
The puffer is very pet-like.
How can you ask a question like that? That's a hard one. I would probably keep a single full-sized tigrinus in a 300 gallon. If we can keep multiples of the same fish, maybe a school of Cichla in a large tank or a breeding pair of Parachromis managuense.