Chemi clean. In fact I just dosed my tank today. I seem to get cyano every time there is some sort of major change. Got it when the tank was new, then a few months after when I added a sump, about a year after that I had a major alk swing from running out of 2 part and not noticing. Then everything was stable for about two years and no sign of cyano but I just moved and now its back full force. Chemi clean has worked every time in the past so Im confident it will work again this time. I have had luck running about 3/4 dose for 3 days followed by a 30ish percent water change.