Low nitrate and high phosphate may happen in small tank with bare bottom. I had a same issue in my Reefer XL200 (total 53 gallon), and resolved by increasing biological filter in the sump while reducing skimming power. GFO helped to reduce phosphate but it should be slowly. I am using Nyos phosi-ex. It's slower than Rowaphos.oh also sorry it's a 40gal nuvo aio