I really wish I knew where my cyno comes from. On my 65 gallon, it hits my tank about every 3-4 months like clock work. I kill it with ChemiClean and my tank is good for a few months. But then it comes back. I have changed so many things over the years, I seriously have no idea what's causing it. Changed brands of food, rinse frozen food, changed brands of lights, even the fixture at one point (old one died), increase water changes, increase flow, add HOB fuge, add a reactor to house Phosguard.
So far nothing has prevented it from coming back.