Usually a cell dies when enough debris flakes off of the plates to short out the bottom of the cell. The cell quickly discharges itself through the short, giving the dead reading.
An equalization charge ensures all cells have a chance to reach a full charge. It won't do any good if the bad cell is shorted and discharging itself. Epsom salts or other additives can dissolve hard oxidation deposits that reduce a cell's capacity. But oxidation usually affects all cells fairly equally, if only one cell is dead while all the others show a full charge, the bad cell is shorted, not oxidized.
I'd bite the bullet and just get a new battery. Trade in the old one for the core credit and it will be properly recycled, not thrown into a landfill.
If the gravity is flat dead in one cell is probably all done. If it's just lower than the others the most I would do is an equalizing. Even if it did recover I might not have much faith in it. If a battery has some variation in cells it's normal to equalize but not often helpful if just one is a lot different. If you have room you may even want to move up to a 27 29 or 31 if you need the capacity. If you are always or mostly plugged in that's not an issue