Azilla,
Great that you're using RO
Was the rock now in the tank cured before it was put in, or is the rock still curing? You might have a lot of dead stuff on the rock that is producing phosphate that is fueling the cyano.
Does your tank have enough water flow to all or most portions of the tank, including the back?
If you have mechanical filtration, do you clean it weekly or more often? Crud that builds up in foams or other filter media will also fuel cyano outbreaks.