I am also battling Cyano right now and nothing I have tried to this point has worked. I also could probably change out my 2 year old ro membrane but my tds meter reads 0 after Di resin so I can't see how this could be the problem.
Unlike the previous posts I know what brought on the Cyano, it was a die off due to a major alk swing I had about 2.5 months ago.
I have blacked out my tank for 4 days and slowed it but it is now coming back. I'm willing to try most things but not any chemicals like slime away, I do know people who have used this and had good results I'm not willing to take a chance.
I have been doing 12 gal water changes a week and syphoning it off my sand but it comes back. My nitrates are at 5 and phosphate at .03
Maybe I will try the special blend and see what happens