If there's symptoms all the fish need to be treated, don't go crazy and start throwing meds at them yet! Watch them for a few days and go from there.
When fish are younger they need food with higher protien but as they age they don't require that, but even that it's where the protein comes from that makes a difference. Compare the labels from foods like NLS and Hikari and you'll see a huge difference in the ingredients. Trust me make the switch and within 30 days youll see color and health that will amaze you! I've kept discus for quite a few years fed exclusively on NLS, mine were are purchased from Jack Watley so they were a little more fragile than some of the local tank raised.