diatom problem

Hi all, just joined and posted my tank specs in the forum provided. I have at the moment a FOWLR tank but would like to turn it into a reef tank in the future. I have been dealing with a diatom problem from the start. I'm at the 7 month mark and am still dealing with it. Since I got my RO/DI unit I have been doing water changes about every 3 weeks. So far 3 changes for a total of 45 gallons. My live rock has turned brown. Initially the brown could be blown off by hand but not at this point. The brown on the glass and sand can be brushed off easily. The rock is slowly getting what I assume is coraline algae. It appears as small ruby colored growth. Also the tank is located in a room that gets a lot of natural light. This time of year it even receives some early morning direct sun My questions are could the sun light be the problem? Will coraline algae be able to overtake the brown on the rock? Could the more frequent water changes (using instant ocean salt) be causing the diatoms to appear faster than before? I am using a reactor with Phosban. Sorry for rattling on. Thanks for any advice you can give😀