Just curious about your feeding. I have a feeling that you are giving too much food. General rule is that all food should be eaten within two minutes or so. Any uneaten food that gets to the bottom will rot. Another way to judge is to look at a fish's eye. Usually it is approximately the size of the stomach. If you ever fillet a fish for dinner, check it out. One last note, the more you feed a fish, the more he poops. The more poop, the dirtier the water. The cories, being bottom dwellers, are closer to the poop and excess bacteria and are usually the first to die.