Is this going to be the only fish in the tank? If not, what tankmates? What is your water like?
I'd recommend pictus or synodontis petricola. Any catfish that grows larger than about 6-7" is too big for this tank IMO. I had a lone petricola for many years, that was in different setups I had, ranging from a mbuna tank to a fully planted tank. Was a great fish.