Your not gonna like the tang police that come in after me, but my hippo can clear and 8 foot tank in literally 3 seconds, let alone with a sailfin and what looks to be two other tangs.
I said that to say this... Your bioload is off the charts. You have four tangs in an 80 gallon tank, two of which call for a 180 or more. This is probably your primary problem. Your phosphate are going to be high as well as your trates which will cause any and all algae to take off. Any dead spots and wham, cyno. To eliminate it, your going to need to decrease your bioload, feed less, and do a higher percentage of water changes. Good luck, hope you can part with some or one of those beauties.