Finding nitrates

[Myka]

When carbon dosing, the bacteria use both NO3 and PO4. The system will usually get to "0" NO3 and still have remaining PO4 because the bacteria use many more times the NO3 than the PO4. Some people claim the Redfield Ratios, but others say that is not quite correct. Point being, you need both in the system in order for them to lower. This is why a lot of people who carbon dose with VSV or biopellets will still use GFO. If you use ZeoStart, you're not supposed to use GFO as they use the opposite theory - instead of removing remaining PO4, add NO3 so that PO4 (and the NO3 you added) can be lowered by the bacteria. What is in that reactor in your sump? Is it GFO?

There's a few possibilities here regarding the rock in the sump, the GFO (if that's what it is), and inaccurate testing. Essentially, if you like how the tank is looking, leave well enough alone.