Ur going to have low nitrates even when u have diatoms because they actualy feed off nitrates and phosfates I had a huge diatoms problem I thought it would never go away because I had a big problem with my diatoms.. I have a 125 gallon and about 130 lbs of live rock... I have a couple questions... what lighting are u using and when did u change the bulbs last? What sea salt do u use? Did u know some sea salt believe it or not have phosfates.. what do u have for clean up crew? One of the most important things is if u use ro
water or do u use tap water? I filled my tank with tap water and that was one of my bigest mistakes.. I started using ro
water changed my bulbs and changed my sea salt and added a clean up crew.. so please answer those questions so I can further assist u.. hope this helps..