I'm guessing that cows aren't part of the natural discus diet. If the natural diet of discus is primarily plants and invertebrates then this could be hemochromatosis. Most invertebrates don't use heme oxygen carriers in their blood so they are low in iron. Vertebrates have heme in hemoglobin and myoglobin and thus much more iron. For birds, where this is a very big problem, species from tropical zones are most susceptible. Tropical regions have low iron due to high rainfall washing minerals out of the soil so scientists speculate that birds from those regions evolved to be very efficient at iron uptake. They have to be fed low-iron diets to avoid developing hemochromatosis.