Convinced of a link between GFO and Cyano (and not the good kind)

[Skim]

Well just thought I would put my 2 cents in. I had Cyno start to appear and this is what worked for me. Pull the GFO and GAC get some Seachem Purigen like 1L and pick up a few of the Seachem " The Bag" works perfect for the Media as it is fine. You can put it in the sump or better would be a small Canister or if you have an AquaClear hang it on the back or side. I did this and within the first day you could see the Cyno start to disappear and your water will sparkle. This stuff lasts quite a while and can be regenerated. It rapidly removes Organics from the water.
One other thing maybe I missed it but is your skimmer working as it should?
The other thing maybe lights old tubes or bulbs if you are use them.
Give the Purigen a try, its simple safe.
The other thing I just thought of that seem to help also was Brightwell Aquatics-Microbacter 7, with a large tank you may need to get the 2L bottle.

Good luck.
All the best.

Mike
P.S one other thing you can try if you use the Purigen is cut back the lighting time or shut them off for 2 to 3 days. That should get ride of the stuff real fast.