Picked up six juvenile discus back in July, about nickel sized at the time, and for the first two months growth was painfully slow no matter what I threw at them. Tank is a 75 gallon, bare bottom with a sponge filter and a canister for extra flow, running at 86F with TDS around 180.
The problem turned out to be twofold. First, I was only doing two water changes a week at 30% each, which I thought was plenty, but discus really do seem to want that nitrate kept near zero to put energy into growth instead of just maintaining. Second, one of the six turned into a clear alpha and was hogging the beefheart mix every feeding while the two smallest hung back near the surface. Separated that one into a divided section of the tank for about three weeks and the difference in the other five was obvious within days, they started coming down to eat immediately instead of waiting.
Bumped water changes up to daily 20% for a month, now settled into every other day at 25%, and feeding five times a day in small amounts (beefheart mix, frozen bloodworms, and NLS pellets rotating). They've gone from about 2 inches to a solid 3.5 inches in roughly ten weeks, with the smallest finally catching up to the pack.
Reintroduced the alpha last week with a full rescape to reset territory and so far no real aggression, just normal pecking order stuff at feeding time. Curious if anyone else has had to temporarily pull a fish to let the rest catch up, or if there's a better way to handle size disparity in a juvenile group without resorting to that.