Wow! Talk about confirmation!
I think that you have put two and two together. I agree that STS alone is a poor substrate and as a soil cover okay, but not the best. Oil Dri is in same category as STS--baked clay.
For the potted plant tank, though, STS and Oil Dri might be perfect! They have clay dust and porous crevices for bacterial attachment and colonization. Associated nitrifying bacteria, as they use oxygen, create the MILDLY anaerobic conditions for denitrification even in environments that don't seem anaerobic to us.
Others viewing my results have questioned whether the STS is taking up N directly. I don't think so. First nitrates are negatively charged, so they do not bind to soil particles. Second, plants prefer ammonia, so they aren't going to take up nitrates unless they absolutely have to. I know of no chemical reaction that would convert ammonia to nitrates. I believe what we are witness here is biological process.
Third, any ammonia binding to soil particles is a one-shot process. The NH4+ cation would have to compete with other cations for a limited number of negative binding sites on clay particles (my book, p. 126, Fig VIII-3). In contrast, nitrate respiration is a continuous on-going process. It never stops. Just as we use oxygen for our respiration, these bacteria are continuously drawing down on nitrates for their respiration.
I didn't think a scattering on the glass would make that much difference, but maybe it can.
I cleaned up and brought Tank #9 with 30 ppm nitrates inside where it will have lighting and temperature similar to a "Twin Tank," #7. My concern was that plants in #9 didn't have maximum growth because the lighting and temperature outside were not as favorable as indoors. Attached is picture of two tanks. I tried to make tanks as similar as possible-same water, lighting, fish load, etc. The only big difference is that there is no STS in #9. Starting nitrates for both tanks was zero on 9/12. Stay tuned!