[bertoni]

Okay, let's go through the basic steps. I'd check the nitrite level, because nitrite will confuse nitrate test kits. That's a low probability event, though.

I'd guess that you might want to increase the NOPOX dose. It's basically a mix of vodka and vinegar, and that dose might not be enough. I suspect that cutting back on feeding will help a lot. There's no need to worry about the Redfield Ratio in this context, since you're not feeding phytoplankton, and denitrification can occur without net phosphorus uptake.