My pops has a tiny sand/gravel quarry on his farm. When I say tiny, I'm talking less than 1/2 acre. It's only 5 or 6 feet deep at most but has fresh water from a stream feeding it through the sand deposit. About 40 years ago an elderly neighbor caught some green sunfish from the stream and tossed them in the pond. In a couple years it was plumb full of them. We brought adult bass and crappie from another neighbor's pond, but they didn't make a dent in the sunfish. The bass got up to 4 lbs. though. Then it winterkilled, and somehow we got a pond full of bullheads after that. I don't know how they got there and I don't have any idea how to get rid of hundreds of 5 inch bullheads...