Yah, if you have food hitting the bottom, you are feeding too much. Instead of getting fish to eat it, just feed less. That said, cories are a great addition to most tanks, IMO. However, they must be fed also. They shouldn't really be kept to just "clean" things. They are not scavengers and must be fed specifically with foods suitable to them (algae wafers, shrimp pellets, bloodworms, fresh veggies, etc.)