Nerite Snail- ?oedema / Epsom Salts

My eldest nerite (almost 5) spends a lot of his time on his shell or on his side, with his flesh protruding - seemingly out of choice as I've noticed him flip himself that way in recent years and make little/no effort to rectify his position even if close to a hard surface to enable that. A quick internet search suggests oedema seems quite common in elderly nerites.
I have a photo here of him in his favoured location (foodpot), where he's been lying writhing for a few days, plus a terracotta ring that I put in to help him climb onto if required.
I'm wondering if a few grains of Epsom salt dropped in to the centre of the terracotta ring might help ie somewhere that he could venture to and know there's a source of something that might help his condition? Presumably that would be better than sodium chloride (aquarium salt). I have both at home.
I wouldn't want to do anything that might upset the other fish (espei rasboras, cardinal tetras), though - although I've given the occasional fish an Epsom salt bath, I've never had salt of any sort in the main tank 24/7.
Thoughts welcome; thanks.

Thanks, Sue.
On reflection, I daren't chance it - although an increase in GH wouldn't be problematic for my very soft water, I'd never forgive myself if it resulted in my nerite dying.