Diatom is caused by the presence of silica in the water, substrate, and sand. Since the tank is matured (not sure how long you have the tank), the silica has to be be introduced from the tap water, water change. You can get Phoslock to remove both phosphate and silica before doing your WC.
Not sure if I agree ... I have 2 tanks. The one with the Eco-Complete substrate is covered in diatoms. The one with the sand substrate has no diatoms. WC for both tanks comes from the same source (tap).