Philosophos

Adequate fertilization is not going to hurt your discus. Plenty of people (including some very reputable ones) keep highly fertilized tanks with healthy, spawning discus in them. Warm water species really don't have the same NO3 sensitivity as trout and such; the lowest LC50's for hobby fish start in around 100ppm. It's things like NH4 and other complex organics correlating to nitrate release from fish that you'll find to be the problem.

Meanwhile skipping out on fertilization and CO2 while using 1.8wpg of CF is going to land you with nutrient deficiencies showing in your plants, as well as algae taking advantage of the situation.

Give then you're running discus tank, you're probably doing frequent water changes as well. This makes your tank a pretty good candidate for EI dosing. I would opt for compressed CO2 or light reduction my self, given that you'd probably need to run a double or triple batch of DIY CO2 along side excel dosing, which is going to cost you more in the long run.

Reducing your light seems sensible in this situation; you don't have any plant species right now that need high CO2 levels or great distribution. You could grow those plants with minimal effort using 3x 48'' T8 for a total of 96 watts. This low level of lighting would allow you to do 50% EI dosing given the slowed growth.

-Philosophos