The African Butterfly Fish don't get more than @ 4" long.They do have a very big mouth though. I wouldn't really expect them to take a hatchet though do to the hatchets body shape.Plus ABF's don't go below the surface very often. they are designed to swim just under the suface.The only time mine went under the surface was when it was playing in the water of the filter outlet.then it would get in the current in the back of the tank, lfoat backwards in the current and then swim @2'' down and start allover at the back of the tank. He would do this for 15 minutes some times.Unfortunately I did find him on the floor one morning:swear: because I didn't have the plastic strip on the back of the glass hood tight enough.That is corrected and I will be getting another soon.If you go with the ABF don't make the same mistake that I did. Secure your hood tightly.
Also, I had him in with a SA dwarf cichlid tank so I didn't do anything special as far as water parameters go. They do like floating cover to "hide" in. Also, www.liveaquaria.com recommends a 6.9-7.1 ph but my water is always @7.4.
Hatchet fish are also surface fish. The are designed that way with their mouth on top of their heads. one of the coolest looking fish I have seen in the lfs.