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?