I'm not familar with PFS but looking at your tank and regime, and narrowing out why its happening, a likely suspect is substrate.
AS far as feeding them.. Assuming you are feeding no other foods.. I would feed one cube per 2-3 fish per feeding at this size. So really 3 cubes min. per feeding. But at this size, if you want them to really grow to potential I would feed at least 2 times a day, possibly 3 if they are aggressive eaters. 2 cubes a day for 8 discus and no other food is really not enough.
hth,
al
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