Have you tested your silicate levels? Are you using RO
water? You can still have algae with a 0 reading of phopahtes. I'm not an expert but something about nitrates and phopahtes being "locked" up and causing the issue.
Don't quote me on it. I thought I read that somewhere.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley