Cyano wont die! Need ideas.

So for the past few months I've been noticing cyano on my sand. I checked all my params and nothing even showed a blip on the rardar. The tank is in the basement and gets no natural sunlight. I run GFO and carbon. I have a skimmer rated at almost double my water volume. I run AI SOL LEDs for 8 hours a day and no lights at night. For flow I have 3 power heads and a large return pump combined they are moving about 5,000gph in the tank. I feed every other day a small amount of mysis that I wash in ro/di water. I do 20% water changes every other week and sometimes a 20% water change once a week just for fun :D. My tds always reads 0. Every few weeks I take a small pump that has a 1/4" line attached to it and I go around to every rock and just blast ever little nook and cranny on the rocks. Most of the time the rocks are clean but every now and then a little detritus will come out.



Two weeks ago I turned my lights off for 3 days straight and wrapped the tank. When I turned them back on the tank looked a lot better but I could still see some cyano.



I'm not sure where this stuff is coming from or how's it's managing to stay alive. Anyone have any ideas on what is causing it or how to get rid of it?