I didn't read back but do you have a good quality GAC in a reactor going? I would change this out and keep refreshing it until the NO3 goes down. Do you have anything in your sump like bioballs or ceramics that are known to cause huge increases in NO3? Also, a DSB that is not getting enough oxygen can cause NO3 to rise.
Dave.M
I do. I have a BRS duel reactor running carbon and GRC. I also have a filter bag in the sump with additional carbon in it No bio balls. I do have lots of additional live rock in the sumo.