I am not completely sure about the science behind this, but when I was battling cyano, I did all the usual things, improved skimming, less nutrients, cut back on feeding...you know all the previously suggested things. And then I tried lights out for 3 days (my only corals at the time were a leather and a toadstool). Gradually lengthened lighting cycle over a 2 week period and within a week of the lights coming back on the cyano was gone.... Do I think that is why? Not really, but what I do think is that by having the lights off all of my other treatments had a chance to "catch up". Just my .02, but it worked for me and if for some reason the cyano should come back....I will do the same thing again, start with most commen suggestions and if they don't seem to be working, go with lights out. As far as hurting the fish, they are NOT photosynthetic so lights out won't hurt them. The corals will probably also be ok. I dont think I would go more then 3 days though.