If you look at Red Sea's info sheet on NoPox for algae control (by eliminating nitrate and phosphate), it states that for it to work properly, a "fractionator" (protein skimming device) is neseccary for it to be effective. Do you have the ability to run an effective protein skimmer on your fresh water tank?
Fractionation removes many TOC and nitrate precursors.ahead of filtration, and is used by salt water hobbyists and pond keepers where large water changes are expensive, and (in the case of salt water keepers) a PITA to mix up.
I have used fractionation, on fresh water and found it effective but......bescause fresh water is less dense than salt water, fractionation is harder to achieve, needing large pumps to fracture the air water interface enough to create foam to waste.
Below is a DIY fractionation unit I used on a freshwater Koi pond.
koi pond fractionation
As you can see, it worked for me, eliminating some maintenance on the pond by dropping waste foam on the ground.
But as stated above, the pond was also heavily planted (with marginals) like papyrus and water hyacinth that also ate nitrate like a sponge.
At the moment I do not use fractionation on my tank (similar size to yours), but use lots of aquatic and terrestrial plants growing from the tank and sump/refugiums surface to reduce nitrate,....but still do every other day water changes to hold nitrate at low levels.

There really isn't any substitute for water changes and maintenance for nitrate reduction