Just an update. The cyano has disappeared. I'm sure it's still in there but I can't find any. I haven't done a water change for a long time (I think I only ever did one on this tank, maybe two). I'm going to take this down and move everything to the new tanks I'm setting up, so no need for a water change now. I'll be selling this, and if I can't I'll keep it around for isolation on new stuff.
I should probably clean the glass.