I'm a newbie pond owner. I have a small pond, about 1/2 acre, with some catfish and minnows. I'm not sure exactly what kind of catfish I have, but I think I have a few types. I've had the pond for about 7 months now and all was fine until recently I noticed the water would look a bit dark at times, kind of like it was muddy. And today, about 20 catfish are dead on the surface, with a few that are moving. We feed the catfish once on a daily basis and have a pump that circulates the water. What could be causing the fish to die? What should I do?! Please help! Any info is much appreciated!