Epsom salt should only done as a dip, not added to the tank. To treat SBD
you can make a tub of a epsom salt bath and put the fish in it for like half an hour...I would not advise just adding it to the tank or letting any fish sit in it for longer than 30 minutes.
Ideally this DG needs seperated from the rest of the fish so he can be fasted/treated properly.
If he won't take small peices of blanched shelled pea you can try soaking them in garlic juice first.