If I didn't have a piranha in my 37gI would set it up marine FOWLR with a dwarf lionfish or a frogfish (aka angler fish). The frogfish is my favorite you could put one in that tank if you got one of the smaller species. Don't expect a lot of action in the tank though, but really is there a more morphologically interesting species of fish?
Here's a video of the angler fish using it's fishing appendage to lure prey and consume it: