I used it for some large wild caught clownfish. It's not something I would do anymore really.
You can't just FW bath and put them back in the tank, or they are immediately reinfected.
Hyposalinity in quarantine is the way to treat fish if your tank hasn't been years established.
http://atj.net.au/marineaquaria/hyposalinity.html
Tough to do, you need a very accurate refractometer.