I used maracyn for cayno a few years back in my 38g reef tank. I know alot of you will think im crazy, but i never had any problems and the cayno was completly gone! I used the "freshwater" maracyn (forgot who makes it) I pulled my carbon, dropped 1/2 a tablet in my HOB filter for three to four days and it was gone, never to return. After it began to die off, i did i large water change and i syphoned out as much as i could.
I don't know if this is completly safe, seeing it kills some of the benificial bacteria, but just wanted to let you know of my personal experience using maracyn.