I encountered the same problem using Kent Phytoplex and Zooplex in our 90g (dosing slightly under bottle directions). Nitrates were never above 5 ppm before I started dosing those products. I was using them to feed a flame scallop. I later discovered those products are not appropriate for a flame scallop after reading an internet article about particle size and ingestion vs. digestion (can't find the link, although it did recommend Two Little Fishies Marine Snow as an appropriate product). Our tank had been running for almost 18 months before I started dosing, so I wouldn't think tank maturity was an issue in our case (even though 18 months still isn't all that long). Needless to say, I'm still fighting to get the nitrates back down... But, I'm growing macroalgae faster than ever!