i would start by reducing the turnover of your water change system.
Any additives you put in the water column are just going to be wasted otherwise, and with a tank that size, thats a lotta waste.
not only that, plants love nitrates. discus can tolerate them in small quantities, 20ppm easily so dont be too worried. Aros are even hardier and 40ppm is no worries.
reduce your surface movement as much as you are comfortable with, just slight ripples should be fine as you have a huge surface area already. keeps more natural co2 in the water that way.
Only then start looking at lighting, CO2 injection perhaps, and ferts - if even required.