The ideal quantity of Sand in a marine tank

Hello, I hope that someone can help me out in giving me an idea about the ideal thickness or high of the sand bed in a marine artificial coral aquarium. I'm straggling to maintaining the phosphate & Nitrate levels at an adequate levels & I've been told that the sand bed I have is too thick & that Nitrate is accumulating under it. I used to use lots of stars to keep it clean but unfortunately these are dead(mostly eaten by fish). How many centimeter thickness is an ideal sand bed? is there a standard to the sand bed

I'm using the sump, protein skimmer, phosphate reactor, Open filter, UV sterilizer

I regularly add Prodibio Biodigest Pro & stop ammo pro (3ml every 4 days) in addition to Red Sea NO3 PO4-x

It's an 1,600liters tank.