There are Madtom catfish which usually stay under 6" in length and resemble bullhead. The only truly tiny catfish that I know of are Hara ssp. such as the Hara jerdoni.
Erethistes jerdoni could be exactly the fish you are looking for. What temp are you keeping the tank at? They like a tad bit cooler water with lots of oxygen
Erethistes jerdoni could be exactly the fish you are looking for. What temp are you keeping the tank at? They like a tad bit cooler water with lots of oxygen