I second the great jumpers, but disagree with the "Not exactly aggressive". Mine are mean mofo's.
Do not put anything that can fit in their mouth that will swim near the top. They're top dwellers and will devour it. I had 6. One "disappeared", two died during a power outage when I was not home. The other 3 are fine. They're amazing fish. They'll stick near the top, ensure you have a solid lid with few holes. I've a Aquaclear 110 HOB filter on their tank so the spray off that is enough to keep them from jumping out there but with a smaller filter it might be worse.
On the aggressive level - they'll kill anything they can and eat it. They'll eat flaked food too though (Mine eat anything, flake, frozen, live). I bought some Hujuta Gar to go with them - don't do that. Mine were ~3-4", and they just snapped them in the middle and slowly devoured them, despite the fake the Hujuta's were 1-2" larger.
On the other hand, fish I've kept with them and had no problem with are:
Five banded barbs, cherry barbs, tiger barbs, platnium barbs, odessa barbs, panda barbs, clown loaches, yoyo loaches, some unknown loach I got from eggs that were on a plant from my LFS, rainbow shark, ropefish, multitude of different plecos, cory cats, upside down cats, some mystery cat, both black and white skirt tetras, banded leporinus, raphael cats, ghost knives, and leaf fish. Edit: Angels are safe too.
So summed up, pretty much any fish that is middle to bottom dwelling or much larger than them. They don't look it but they're surprising fast fish as well so don't get a top-dwelling fish that you think could outrun it - odds are it can't. Their jaws drop down, so don't take what you see for their mouth as how big their mouth is.
Side note - their nips hurt!
