Yes, that medication is the correct one...the one for saltwater fish, not fresh.
Yea, links to stores are not allowed, but...I guess you can say this is one of the situations that a fish's life rules out.
You can not add medications to a display tank.
The quarantine should be cycled, and if you are just setting it up, its obviously not cycled. Use 100% water from the display to fill the hospital tank. It will be extremely difficult to maintain water quality in an uncycled tank. Be prepared for many water changes, daily. Keep the lights off in the quarantine but still have room lighting. You can turn tank lights on about an hour before feeding, and during feeding of course.
Is this fish still eating?
When you transfer the fish to the quarantine, do not net him and remove him from water. Instead corral him into a ziplock that is filled with water and submerged in the tank. Since your hospital tank will be the same a your display water, no need for acclimation. Just make sure the temp in the two tanks are the same.
Likely there will be no problems with your other fish, as long as this problem with the tank is not due to poor tank environment.