It took me about 9 months to get the nitrates under control in my 90 gal
., but I didn't have any live rock or a protien skimmer. One thing I know of that didn't help was using CC for a substrate. I don't know what you are using, but that could be a problem. Also I was using aqua medic test kit, that I didn't think was very reliable. It read all the way up to 100 ppm
! And was very open to individual interpretation. It also may just be a time issue. HTH
, I'm not an expert but that was my experienc.