Tell your friend that at least he should add some potassium to his tank. Fish food doesn't supply much potassium, and, with plants competing for potassium, fish can get into trouble. I know this is true for guppies. It may be that Discus can handle low K better, but guppies get whitish growths on their skin if the K gets too low. Adding K makes the growths go away after a few days and the guppies get more active and look better. If you don't add K when the fish get the growths, you will loose most of all of the guppies.