Before you treat the tank with antibiotics, which will kill your cycle in the tank causing huge ammonia spikes try and get to the root cause. Getting to the root cause will prevent it returning, just treating it will not!
If you know your nitrates and phosphates are high do more frequent water changes - try increasing to 20% twice a week rather than 20% once a week etc.. The test kits for phosphates are fairly inaccurate, most cannot get accurate enough, you may only have 0.25ppm showing but i bet you are closer to 1ppm. I would recommend getting a
phosphate remover, if you cannot afford one like me just get the media that goes in them stick them in a nylon stocking and hang somewhere in your tank (i hang mine in the over flow box). Nitrates and phosphates in a reef tank should be 0ppm!
Cyano bacteria likes low flow areas, what
GPH do you have flowing through the tank right now? For my 75g i have 2x koralia's 4 and 2x koralia 1. You can never have too much flow in your tank.
How much do you have for
CUC (clean up crew) i would recommend adding more snails and hermit crabs, they might not eat the cyano-bacteria but they will help stir the sand/crushed coral keeping the rocks etc... cleaner (again preventing nitrate rise).
Whilst doing the water changes i would syphon up as much of the cyano bacteria as possible, its comes off the rocks and sand bed easily.
I have been there in 2 tanks now and got rid of it but getting to the root cause of the problem, i have never had to use chemicals to get rid of the problem, all of the above are what i used to cure the problem. 90% of the cyano bacteria found in tanks are from inadequate water flow, increasing the water movement by adding a
powerhead (like i said earlier) and maybe moving rock work around to improve water movement will help.
Good Luck
Anna
EDIT: i have read your other posts with your ick/velvet problems!