You mentioned your substrate is rather large correct? One thing about discus is that they tend to like to graze alot. With large substrate that will be alot harder for them and you will lose alot of food that way. Sure they will attack food when it hits the tank but they tend to slow down then graze after the 1st intial feeding frenzy. If you want to have substrate I would suggest a thin layer of pool filter sand as it is small enough both the food and poop hit the top and do not settle. It will make any spot cleaning much much easier. It would be best if you could up your water changes a bit as...even though water may stay within good parameters. Young discus produce a hormone in the slimecoat that does inhibit growth. And changing the water frequently reduces the amount of hormone within the water column allowing for better growth. Jack Wattley found that even hooking up growout systems to a centralized filter that the smaller discus within the system tended to slow considerably in growth. All of his growout tanks are now on their own separate filtration.