So I have a high nitrate problem currently. My nitrates always seem to be around above 40ppm even after a 50% water change. However, my tap usually has 5-20ppm nitrates depending on the day. So for stocking it's 3 gulf killifish each about 4 inches. Aquaclear 20 filter, 10 gallon tank. 0.25/0/40< are the parameters. For plants 1 Amazon sword, 300 duckweed plants, and 15> stalks of egeria densa. Substrate is caribsea supernaturals aquarium sand that is supposed to absorb nitrates. One piece of driftwood. What do y'all think I should do about my high nitrates and what could be contributing to my problem? the goal is to have 20ppm nitrates the most, so I can add 3-5 grass shrimp and keep my fish healthy. I'm also dosing Seachem Flourish as instructed by bottle once per week 0.7ml.