I have read information that pointed to a possibility of improper lighting contributing to cyanobacteria. It seems that cyano prefers a spectrum of light different than optimum for things like corraline. I had been using 9325K GE AquaRays (spectrum peaked at 545nm which is perfect for Cyano) and within 2 weeks of switching to a Coral Line 10,000K it vanished. I didn't do any major water changes during that time, just enough to keep the nitrates in check, with tap water, not ro/di (FOWLR Tank). I've never seen any of it come back running the 10,000K lights. Just a thought, really it's a combination of things, but if everything else fails to work, check your lights, they may be of the wrong Kelvin or reached their end of life (flourescent lights shift in spectrum over time, they still produce adequate light, just the wrong spectrum). Also, when you buy lights, check the LFS and look at the package, many of them have a graph that shows the peaks in the spectrum of the lamp (one for each phosphor I guess) and make sure that none of the peaks are around 545 nm.