You might try one or all of what worked for me. I had a bad cyano
problem for a few months. Im not sure what knocked it out but i got an RO
unit(I was buying RO
water at the LFS
but used treated tap for top offs occasionaly) Ditched my HOB
filter. I siphoned off some sand and replaced with a new top layer. Scaled back feeding. And what I think did the most good was changing my bulbs. At the time my bulbs were 8-10 mo old. A week after the new sand, 3-4 10-15% water changes, and new bulbs, the cyano
was gone. My nitrates and phosphates went down and I havnt seen any cyano
in my tank since. And my corals are looking better than ever too.