The dangerous thing about doing that is that if you just take a new empty tank and fill it with water/salt, and you put a filter on it and even if you put live sand in, once you put the effected fish in there and feed it, it would trigger a cycle. This would generate ammonia and it's a sure way to just plain kill the fish.
If you want to set up a hospital/quarantine tank, you have to take measures to assure that no cycling would be involved. So, use water from the established tank that you have and use sand/substrate that is in that one, too. Also set up a filter of some kind...even a hang on filter...but use some medium like filter floss or similar substane from an established tank. If you don't have any, get some from an established tank from a fish store or a friend or even post in these forums for some from a nearby hobbyist. And, make sure the water in the quarantine tank is well oxygenated.