At the hatchery I worked at, we cleaned out some old INDOOR tanks that had collected some stagnant water in them, probably from rain leaking in or something. They hadn't really been touched in 3+ years and the hatchery has never had bullhead other than a dozen or so wild caught in the display area, and there were about a half dozen 5"+ bullheads. No moving water, no filtration, no added food. I have absolutely no clue how they even could have been introduced there as it was inside. It's not like a bird could have carried the eggs as can happen with bluegill. Bullheads are VERY resilient fish 
