The coolest "predator" tank I ever saw was a 20 gallon corner tank, with one big peice of driftwood, and a 2 foot ropefish. Not a mean predator, but an AMAZING display.
I had a leaf fish once. They was only able to get it to eat small fish. Tried to feed it other types of foods, but just ignored them. It loved tetra's and guppies. I first had it in a 10 gal. then later a 20 gal. Then started to sell off some of my tanks and relocated into a 55 gal. tank.
Just Found some Old pics and wanted to post. Here is my ctenopoma after just eating an OTTO I put a few in to get all the algae in the tank. The damn Otto was about 2/3rds his body length. Funny thing is he beat everything else to the Ottos. I wasn't sure what to do as the Leaf fish went and hid to digest it I guess. My understanding is that some cat's spines are toxic?? could be wrong.
I've got 2 golden wonder killies in a 10 along with a couple of corys... look nice, tear up any brine shrimp, mosquito larvae, etc. I put in there. May have to put a couple of guppies in my daughters tank and try 'em on fry.