I have a redish rust colored algae growing in a portion of my tank on the rocks. Gets a dusting on the glass too if I don't clean it for a few days. It isn't thick like a mat. A brush causes it to come off like dust almost. I know the pic does not show the color well but can someone help identify this please so I know how to treat it? I have been dosing nitrates because they were near zero always. Running about 2 now. And My Phosphates are about .04. PH 2.1. Any ideas? When I clean it off with a brush, it comes right back in the same area in a week or 2. Thanks for your ideas.