I don’t think nitrate of 5+ ppm is causing the browning. Not only is it not very high, your test kit’s margin of error and/or human error might make your readings high or low. When I started dosing for dinos, phosphate additions seemed to lower nitrates, and nitrate additions seemed to lower phosphates. I don’t know if there is a chemistry reason for this or if it has to do with how dinos uptake the nutrients, but I saw it and have read where this is common (at least when dealing with dinos).
If you don’t want to stop dosing NOPOX, why not add Flourish or a different phosphate source and see if you can raise phosphates without significantly lowering nitrates?
