Here is my own view of cyno. Until I went bare bottomed, all my dsb or shallow sb tanks always had a persistant cyno problem. Despite all of the usually suggested fixer uppers. Heck, I ran two 6080 Tunze streams, a huge beckett skimmer AND a large turf scrubber.
I would put that to an accumulation of nutrients in the substrate. Which means, as mentioned above phosphate. I would say that substrate in a tank needs some sort of maintaince, either by hand or many animals of some kind.
In my friends 180, this is what we did. First we removed some of the sand and made it more shallow, for easier cleaning if neccessary. When he ran Phosban, it helped somewhat. He has all of the above listed suggestions, including a large beckett skimmer and huge nutrient consuming soft corals.
What ended up giving the best results as adding a calerpa species to his 100g sump and using enough light to compete with his tank. Presto, cyno gone, red turf on rocks gone. I assume that its consuming the phosphate from his heavy fish load, and thus heavy feeding.
Of course one could have a small fish load and feed a lot less, still run a decent skimmer or large water changes and keep particulate material from collecting someplace.