UVS effect on nitrate levels?

• pH:
• Temp:
• KH:
• GH:
• NH4:
• NO2:
• NO3:
• PO4:
• Iron:
• Dosing: 25 mL Excel, 1 tsp MgSO4, 2 tsp CaSO4, 15 ppm NO3, 5 ppm K, 1.8 ppm PO4
• NO3:PO4 Cumulative Dosing Ratio: 15:1.8 = 8.3

Notes:
50% water change:
2.5 tsp baking soda
20 mL Amquel+
15 ppm NO3
5 ppm Flourish potassium
1.8 ppm PO4
1 tsp MgSO4
2 tsp CaSO4
25 mL Excel

I noticed that after every water change, in the beginning of the week, that the plants seem to show a nutrient shortage, which seems to be potassium. There are black pinpoint holes in the leaves of the L. repens, and the stargrass shows black patches in the beginning of the week before growing out nice and green. I still have some leftover flourish potassium, so I added 5 ppm to the water change. The yo yo loaches are uprooting some of the plants, especially the A. senegalensis, which is annoying to replant, since the stems are so buoyant.

I ran the UV sterilizer for the last 2 days, which still didn't make any impact. It seems to accelerate the decreasing levels of nitrates. Yesterday, I added 7.5 ppm NO3, after levels fell to nearly zero from a level of 10 ppm only the day before. I learned the hard way that if nitrate levels fall to zero for more than a day, the plants quickly suffer, especially the plants which take their nutrients more from the water column rather than the roots (anubias, H. micranthemoides, java fern). That's why I'm reluctant to run the UVS for long periods of time, though the slight green water is bugging me. I'll probably run the UVS for several days this week, and keep a close eye on the nitrate levels.

Interestingly, several hours after the water change, the red on the leaves of plants was noticeably more green. I had waited a couple hours after the water change to add the nitrates/potassium. Usually, I add the nitrates immediately after the water change. I waited this time, because my theory is that the Amquel+ neutralizes/locks up the nitrates if I add them into the tank at the same time. The change in the leaf colors seems to bear this out, though perhaps the extra dosage of potassium could also be a factor.