Crazy..just crazy. I know it worked, and others have had success...I personally shutter at the thought of adding Anitbiotics or any chemicals into my tank.
You spent 3-6 weeks, patiently waiting while your tank cycled, all this time to establish beneficial bacteria...you spent more money than you wanted to spend on LR to give the bacteria a place to reproduce and break down waste...
...and you just killed it all.
I've had my share of Cyano, and while it is really a resilient "bacteria", good husbandry practices will get rid of it. Patience really is the key word in our hobby. Especially when it comes to Cyano and Diatom algae.
I spent the better part of a year fighting Cyano, eventually it went away, never to return (been 2 years now). Did this through strict weekly water changes, Light feeding, Strict TDS testing of my RODI water, and frequent siphoning of the sand bed. I try to stay clear of adding chemicals/Antibiotics to my tank.
Not throwing stones here..just don't understand how you had the guts to put Antibiotics in your tank. Youre Cyano problem must've been really bad! Good luck.