I also have been fighting it. Seems to happen in waves over the years. I don't think flow has that much to do with it as I have a hard time keeping my sand in place my flow is so high, but Cyano
still grows. It has to be a nutrient issue like MrX stated. Try using Polysorb and increasing the output of your skimmer if possible. You have to suck the nutrients from the water some way or the Cyano
will grow. I have heard high magnesium levels can reduce it, but have not tested that yet. Nothing I have found eats this garbage.