Bro... Mine is a FOWLR tank and I'm feeding MP to my 4 puffers, a fox face po and a small lobster. No corals...
The puffers are greedy and they will makan all the MP that I give (not a lot anyway) and the smaller pieces will be eaten by the face face and the lobster so very little uneaten food.
Maybe detritus buildup could have cause the high nitrates but my skimmer also not getting a lot of gunk out...
I'll try to clean my bio balls or remove it totally... Also "clean" my sump at the same time too...