Well, it was later than anticipated but I took measurements of phosphate and nitrate today.
Measured at .5 and 20 ppm respectively. Will try dosing some KH2PO4 and see where that goes.
Would be nice if that makes some things better. It's certainly outside the usual 10:1 ratio and .5 is hard to measure. If the test is just registering some PO4 as .5, might be that you occasionally run out of it, particularly during the height of the photoperiod. You can increase the sensitivity of the PO4 test by diluting it 5:1. I use 40ml of RO/distilled water and 10ml tank water, then run the test as instructed from that mix, but multiply the results by 5. Sometimes it's a little easier to see color differences that way. My Salifert has multiple levels below .5 (I forget what API has). I do the same thing with my Hanna colorimeter.