cory cat epidemic

It seems that every corycat I get eventually succumbs to the same
disease. The only symptom is that the fish loses its balance and cannot
swim or lie on the gravel straight. Eventully the fish dies. All my
water parameters are good, the temperature is 78F and I keep 2
angelfish, a pleco, some tetras and ottos which are all doing very
well. I really like cory cats and I want to have them in the future but
I have to figure out what is going wrong. Thanks for any help.