You Probably do need to provide some supplement. A high light, CO2 injected
tank almost certainly needs supplements.

My tank's substrate is flourite, and I find I cannot do without supplements. I
am looking into supplementing my tank's nitrate as well.

Although Flourite has good iron content, there are a number of other trace
elements a fast growing collection of plants require.

Trace elements other than iron important for plant growth include Sulfur, Zinc,
Calcium, Chloride, Boron, Copper, Magnesium, Manganese, Molybdenum, . All of
these elements are in TMG, and are components in a PMDD mixture. These
elements are also probably present in your tap water. You may be able to get
by without supplementing these; a weekly water change may be enough for you. It
depends on the growing conditions in your tank.

My plants are growing so rapidly anymore that I find I need to consider
supplementing the nitrate level. Fish waste eventually becomes nitrate in the
water column, so in mamy aquariums this nutrient is too abundent. However
changing water weekly and fast growing plants can void the water column of this
important nutrient.

-Bruce