I had bad cyano a few months back. Did daily water changes and it would just come back. I did chemi clean and it worked great, but just for a few days. I added conches and a tiger tail cucumber and they helped but couldn't keep up with the daily growth. What I think finally helped was not siphoning the sand during water changes. I think letting bacteria and critters settle in the sand helped increase my natural bio filtration, instead of constantly disturbing it. You might want to try leaving your sand bed alone if all else fails. The thing I also noticed with my cyano was that it was just on the sand.