Nerite Snail - Ill/dying & Suggestions For Trtmt?

My oldest nerite, purchased in May 2017, isn't faring well. For the past year or so, he's had a tendency to fall off the glass at least daily and, despite being within reach of a hard surface to help rectify his position, "prefers" to lie upside down indefinitely. In the past week, though, he's scarcely moved. For a few days, his flesh was protruding out underneath and to one side of his shell, and he kept twisting and turning his shell in semi-circles as though attempting to align himself appropriately. Yesterday, he got onto and moved briefly on the glass. Today, he's returned to "writhing". Sometimes, this happens during/post-mating, but I'm wondering if he may be suffering from oedema as internet searches suggest is common in elderly snails - it might account for his "preference" to lie on his back.
Other possibilities are that he's ill (but no idea what possibilities there might be) or that he's been inadvertently poisoned. If so, having racked my brains, might any of the following be potential routes/toxins?
* bamboo feeding pot - purchased in June 2019, and both snails have had a tendency to munch it; they munched through the base a month or so ago and I removed the pot but replaced it recently after they refused to eat from the plastic one (and he spent a couple of days munching on its circumference); a quick internet search suggests a cyanide linkage

* wood is getting much thinner due to amount of it that gets chewed
* Seachem Flourish and its root tabs - possible that, on removing the small terracotta pots during tank maintenance, the dissolved root tabs have escaped along with some sand through the hole at the bottom - copper at dangerous levels?
* nano black cones (old packet) - put 3 in the tank a couple of weeks ago.
This evening, I've put some Poly-Filter and carbon in the filter, and removed the bamboo feeding pot and black cones. Any other thoughts on the above or otherwise, and suggestions?

I've not witnessed this behavior in any of my snails. One of the older assassin snails went through a brief spate of being found on his back, but always managed to right himself eventually.
I've not seen any of my nerites bulging out of their shells either.
I have used alder cones in several of my tanks in the past, but not recently (not for any particular reason though).
I tend to use the JBL Pro Flora fertiliser balls, so can't comment on the Seachem ones. I can't imagine they would sell aquarium products without a warning if they weren't safe for snails though.
Sorry I'm not being much help here.

Thanks, both.

He's still alive. There's much less time spent twisting and turning in the one spot but I have seen it a couple of times. Most of the time, he's now back on the tank walls. Occasionally (but no more than has been the case in the past year or so), he's on the tank floor, upside down, with his innards/flesh protruding, but seems to sort himself out after I carefully flip him over with an aquascaping tool.

The one thing that intrigues/concerns me is why neither snail is eating the snail food. I cut up a catappa leaf and lined the plastic dish with it, then placed the snail food on it but it's not been touched. I'd really like to see them (and particularly him) eat, to strengthen him up a bit and hopefully get over whatever has been / is wrong with him.

Thanks, Sue. He's been spending a lot of time on the catappa leaf on the substrate lately, so perhaps he's getting some nutrients from there, and I see both nerites are back on the wood again today after an uncharacteristic spell of ignoring it. The filter and heater are algae-free but that may well be because of the nerites' work. However, I did remove all the pebbles as I hated the sound of the the nerite's shell landing on it when he fell/jumped off the glass, so I'll see if I can grow any algae on them on the one window that might get some daylight on it. Presumably they need to be in water? If so, I'll prepare myself for the inevitable groaning that the water from it, as well as from the fishtank, is the cause of our severe condensation problems...

Thanks. I have to remove the container at night as so much condensation pools on the window sills during the day and overnight that the container would be floating.

He's spent a lot of time lying on his back on the tank floor with his innards protruding but rectifies himself relatively swiftly once I turn him over without too much strange twisting. I caught him "attached" to the female snail a couple of days ago but she was more interested in the wood than engaging with him.

Photo of him now currently on the wood - he's spent the day working his way along it (trail of 'evidence' underneath it) which is a good sign that he's back to his former behaviour (touchwood).

Yes, we have one of those - it's Mr FCMF's favourite toy! It fills to ~70% each morning with grey water which he immediately empties. I cringe when he starts using it at 7am, in case it wakes the neighbours, but he uses it 2-3 times throughout the day - certainly getting the £'s worth of usage.

Glad to hear that things seem to back to normal. 

I've not seen any of my snails do that.