Gulpers can eat fish up to twice it's size. The trick is if the fish is wider and taller than its mouth. For instance my Aro a victim of my Gulper was twicw his size but not deep bodied. So the Gulper got some of it in his mouth and folded the Aro's body in half to compensate for it's length. You can conceivable successfully keep them deep bodied fish. Like Pacu's, Silver Dollars, Stingrays, Tachy's even a Arro if it's much larger than the Gulper. As Aro's mature they become more deep bodied. Just make sure the height (not length) of the body is of tankmates is larger than the Gulper. My 4" Gulper can open his mouth about 3" so the fish's body height should be bigger than that. Another fact to consider is Gulpers aren't aggressive just predatory so Cichlids might be a poor choice. The Gulper would get picked on and wouldn't defend itself and eventually die. I'd say Pacu's and larger Aro's and catfish are probably the best tankmates for them.
