i will definatly go along the way of the catfish, quite a few choices there i especially liked my red tail catfish (sorry forgot the proper name), another choice would be to have a few paroon sharks (aka naja shark) in there, but be warned these catfish can and will eat anything half their size (lost about a dozen 4 inch tiger oscar to the redtail, i know a beginner's mistake)...one final advice look at your tank dimension and chose the fish according to the guidelines that i have found out from many a experienced people ....
1. the length of teh tank should be at least 5 times the fish length
2. the tank width should be at least 1.5 to 2 times the fish length
3. the tank depth should be at least 3-4 times the fish height