Epsom salts are used in treatments of fish where the "bubble" or bouancy is caused by the fish retaining air e.g. anglers, seahorses, etc . It is a substitute for diamox, which is a controlled medication that you need a prescription for (unless you buy from Canada).
I have never heard of it being used for popeye in a salt water fish. That doesn't mean it doesn't happen, I have just never heard of it. I've never had a fish with pop eye so I never looked into it, but Epsom Salts are used with other fish.
If you are going to use it I would recommend a pure unscented form with no added chemicals or fragrance.
If you are treating the fish in a separate hospital type tank IME, usually the problem results from poor water quality. I have had to do 50% daily water changes in hospital tanks replacing the same percentage of medication as the water that was changed. To much antibiotic is very bad for fish as well. Going on to a third week , especially stepping down in intensity of the anti biotic would be a cause of concern for me. If the stronger antibiotic did not kill the problem treating with something weaker will likely make the original bacteria much stronger.
JMO Take it for what it's worth.
Edit: It does appear that people are treating pop eye with Epsom Salts and it has shown positive effects if the popeye is caused by an injury and fluid retention and not by other means.