Actually, large catfish may live in the Amazon, but they live at the bottom, where it would be cooler than the warmer temperatures you'd find at the top of the river. 78 is better than warmer temps for them, and probably matches their conditions in the wild. Warmer temps decrease the oxygen level, while increasing the fish's metabolism at the same time, which could lead to problems. My pond runs anywhere from 74 to 78, and my fish are active and happy.