[Delphinus]

Once it gains a foothold it's really hard to get rid of it, by relying on water parameters alone. One dirty little secret I think most reefers won't admit to, but do secretly use, is the use of Red Slime Remover or Chemi-Clean or similar anti-cyano product. They make your tank go bright yellow and even smell like a banana, you'll think, OMG what have I done, but ride it out. It doesn't stay yellow forever. Turn your skimmer off because it will go NUTS (or better yet, just have it empty back into your tank, because it does depress your O2 levels), and after a couple days do a water change and run carbon to clear it out. In a really bad case you might have to run a second course a week or two later.

Assuming your parameters are good (and what you've posted, they are), the cyano shouldn't come back. If it does come back quickly, it means there's a reason, be it parameters or light or whatever.

However, one "food for thought" item, cyano is very often the result of elevated nitrates. You posted that your nitrates are undetectable. What are you using for testing nitrates, because I don't always believe a zero reading for nitrate.