When I started using NoPox for carbon dosing last year, I wanted to reduce an elevated Nitrate level. The RedSea recommended dosage was “3 mL / 100 L(25 gal).”
This initially worked, but after a while the process got “stuck.” Turns out the tank also required the addition of approximately 0.100 ppm phosphate daily using NeoPhos to keep the phosphate over 0.050 ppm, and allow the process to continue. Overall each reduction of 1-2 ppm nitrate required approximately 0.100 ppm of additional phosphate (some of this total was also the baseline requirement of the tank before NoPox).
While others on R2R have posted similar experiences, others have posted no need for phosphate addition. The reason for this is unknown to be.
While the OP is measuring a seemingly reasonable phosphate level, I would try adding additional phosphate to raise the phosphate level 0.020-0.030 ppm. Maybe there is a testing error or something else going on that is resulting in phosphate limitation.
There may also be a trace element limitation as mentioned, so a couple nice water changes may also result in an improvement.
@Dan_P has made an interesting observation that I also observed. When reducing the NoPox to increase nitrate, at times a seemingly small reduction can result in a relatively large 2-3+ ppm increase in nitrate the next day. I did not understand this, but Dan’s thought of bacteria die off and nitrate release back into the water is interesting, and makes sense.
To supplement nitrate reduction, I have also had really good results using Seachem Matrix in the sump. It’s inert and not chemical based so you cannot put too much into the tank. 1-2 L / 100 gal in a very coarse media bag should produce significant nitrate reduction after a few weeks or so. Over time it’s amazing how many pods and sponges etc. populate the Matrix to form a wonderful aerobic and anaerobic biological filter. I can’t prove it, but I think this also significantly improves carbon dosing nitrate reduction.