I do have a dead spot in my tank... and actually was wondering if it could be the cause. It's a dark, dead spot on the bottom of the tank that is hard to get to, and get's almost no circulation. I've often wondered if something died under there, how I would get it out...I would look for any dead spots in your tank. Sometimes cyano shows up because of inadequate flow in the tank.
This is a DAS aquarium, so it has a built-in filter box to house a protein skimmer, two sponge filters and another pump. It seems like a good idea, except there is a 2" gap at the bottom... I'll see if I can get a small submersible filter and point if under there...
Can you see it on the left?