I don't believe the low nitrates or poor circulation, I think it is a bacteria that you need to kill off and it can be introduced through new plants, fish, etc. I just had it for the first time in a very old established tank and had to use Erythromycin too get the job done.
I noticed the top inch of my substrate (Flourite) had a white substance that was not there before I killed it off but I think it settles in to these low oxygen areas and robs O2 from the plant roots, I was having trouble growing S. porto velho prior to the Cyano but after the treatment it took off and grew like crazy and a couple other plants showed marked improvement. It also only collected on Riccia, Fissidens, and other mosses in my tank. I'm sorry to hear your having this problem over and over, this is some stinky stuff.
I am certainly no expert but these were my observations.