What is killing my snails?

[KrisReef]

Heres one on his way out. Barley stuck to the glass hasnt moved in days looking sad

Heres a dead one in the tank (terrible picture)

And heres 2 dead ones I pulled out today

Maybe they are just getting old and giving up?

 

[Rick's Reviews]

I have had some snails now for nearly 4 years, part of my first cuc, I don't know if I can add anything to this but I have killed a few snails during my water change, not allowing salt to dissolve properly before adding to aquarium.
It appears you have 'all' and more systems for this but could a simple thing like this be happening in your aquarium?
Besides that maybe just lack of algae/ food, try maybe some dry seaweed on a clip or algae wafers to see how they respond to it