It is a tricky problem: With truly zero nitrate+phosphate, corals will starve. As zero nitrate/phosphate levels rise, corals are happy, but so is Algae. If you are nitrate limited (0 nitrate, but not 0 phosphate), you get Cyano problems. So generally: 1: Lower phosphates with GFO/etc, so you are more phosphate limited than nitrate limited. 2: Lower all nutrients in general. Carbon dosing, water changes, etc. Many methods. 3: Manual removal of as much algae as you can. 4: Snails, hermits, and perhaps a lawnmower blenny as herbivores. Yes, snails cannot keep up alone, but with 2+3+4, they should be able to.