Were you thinking salt or Epsom Salt.
Salt would be a good thing if he has an external parasite issue. You might try a dip and see how he handles it.
Either salt or Epsom salt make sure it doesn't have any additives, last night at the store I accidently grabbed the wrong Epsom salts and got the kind with peppermint and rosemary oil for a people bath, oops, but didn't use it for the fish. So just make sure you grab the right kind and double check the ingredients before purchase.
Try a 2tbl per gallon or 1Tbl for the half gallon dissolved salt treated water temp matched, add an air stone if you can.
Let him be in there until he tilts, falls sideways on the bottom or rolls over OR for only 30 min.- you have to watch him the whole time.
Then remove him and he will likely be pale and could be splotchy. Make sure the tank water is nice and fresh. Not Ammonia, Nitrites or Nitrates. He might lose some slime coat, try a 50% pwc an hour or 2 after, vac the bottom. That will just get out any slime coat sluffed off.
If you don't have an air bubbler maybe get a small one and use with a valve you can use to lower the flow if needed. IF you do not use a hang on the back filter. If you have a hob filter then just lower the water a little so it gets plenty of splash, and extra aeration.
You could do the Epsom Salt in the tank until you get the meds in the mail.
Clean fresh (treated) water every day.