I battled a really nasty string algae/cyano/dinos cycle a couple years ago. It was remembering my high school science that helped me get ahead of it. "energy in a closed system is never lost. It can be converted in form but remains constant." So to that end I put in urchins and snails and macros in the sump. The thought was to get them eating the stuff as I also manually removed as much as possible. That interrupted the cycle by getting whatever it was feeding on into animals and export. I did use some bacteria additions, small ones, not sure if they had any effect. Took a while but I finally got the cycle broken and tank cleaned up. Just my .02