The cyano was growing in there from having it DSM for over a year. I filled it up a bit too much once and the little amount completely took off. I have always had cyano issues in my tanks and I don't know why, maybe excess phos in my tap water... I have tried adding more flow (powerheads) and dosing KNO3, nothing has ever really made it go away. I took down my other tank because of it. I will try the peroxide.
Here is a pic from my old tank. yuck.
I don't want to put holes in the ceiling. Might have if it was my own place, but I'm renting