I tried reducing the light to only on for 4 hours per day (that was all I was willing to do, because I have several SPS corals). I didn't see any difference.
Nitrates are virtually zero. I haven't had that problem at all. My sand bed is 6 inches deep. Concrete has a lot of phosphate and I believe that is the problem. The phosphate will deplete over time. Also, my RO unit broke and I have been using tap water for over a year to top off at an average of 15 - 20 gallons per day evaporation. That is probably the biggest problem. I'm ordering a new RO tomorrow. I also just ordered a dozen sand sifting gobies.