If you find by feeding more you’re unable to raise phosphates, I’d recommend dosing something such as sea chem flourish phosphorus. For dosing nitrogen, I make my own stock solution using pharmaceutical grade sodium nitrate from a company named Loudwolf. It’s a lot cheaper doing it that way. A Hanna ULR HI736 checker for phosphates is a must also for measuring low levels. Amazon is the cheapest place I’ve found seachem flourish phosphorus. Here’s a link for making stock nitrate solution. It’s an awesome calculator.
Seachem Flourish Phosphorus 500ml
Seachem Flourish Phosphorus 500ml