Having an issue with brown algae on gravel. Tank is many years old. It definitely disappears when lights. I thought it was Cyro but chemiclean really didn’t help. I did discovered I had a bad cartridge in my RO/DI system and my top off water was full of silicate. I am using phosbond to remove but has had little effect. Any recommendations is appreciated. My nitrates and phosphate are very low.