If you have probloms with algy on your plants the best thing to do is gently wipe it with some paper towel, works for me. As fare as will the flousih hurt your discus, at the end of the day it's another chemical. But they wouldnt sell it if it hurt fish. If I was you sence discus are rather sensitive I would try and stay away from adding any chemicals to the water but if you must I would put less then what you need and keep a very close eye on the fish