[Enigma]

I think I would keep them both separate. That way, if one crashes, the other won't go with it. In the event one crashed, the other might be useful for the emergency storing of some livestock (with a tank divider, or something).

Personally, I really like spiny box puffers and lionfish.

Edit: Sea Goblins are very interesting, too. And, One local shop has a frilly rhinopius right now . . . Which is insane (with a crazy price tag to match).