Greg the goal is to always keep nitrates <10, meaning before each water change. The types and freq of feeding dictates those numbers. But also bacterial counts and slime build up (esp when feeding beef heart) affect growth & health. Whilst discus growth somewhat slows when they hit 4.5" the cleaner the water you provide, the faster they will reach 6"+, and they will be healthier and have better colors and more vigor along the way.