1. Good flow. Nothing should settle on the substrate.
2. Run phosphate removal media such as phosban. I run the media eventhough I have tested my water with a salifert kit and it read 0. The Algae can be using the phosphate as fast as it's being intorduced to your system, and even small amounts of PO4 (too small to register on a kit) can fuel algae and harm corals.
3. Use RODI water. Check the TDS, your filters may need replacing.
4. Use high quality carbon. Don't buy the cheapest brand you see. If you have not heard of the brand before, dont buy it. Poor quality carbon is notorious for leaching phosphates.
5. Rinse all frozen food cubes before feeding.
6. Keep the sand stirred with a cucumber, conch and/or gobies.