The strangest thing happened last week, both of my mystery snails died within just a few days of each other for no apparent reason. The snails were in a 20 gallon tank with 2 fantails, ( I know technically a bit overstocked) I do 50% water changes twice a week and their water quality is always perfect. But about 6 days ago their filter got unplugged somehow and I didn't notice for 2 whole days because the fish were completely fine. The water only tested about .25 ammonia when I tested it after realizing what had happened but the larger of the two snails was laying on his side with his trap door closed and he wouldn't move or eat. I did a 100% water change over the course of the day and everyone seemed fine except the big snail. Over the next few days I was sure that he was dead but the small snail seemed fine. Moving around, eating, etc. Then, yesterday I found him in the same state as the big snail. I can't figure out why they both just died! The water didn't even get that bad and the smaller snail seemed fine up until yesterday. Is there anything else that could have killed them? A new fish was added recently but he had been quaranteened and was totally healthy. What happened?

Oh - I'm sorry you lost your snails.
I'm pretty sure the loss of filtration was what killed them. Snails are super-sensitive to water quality and are affected immediately the water is even a little 'off'.
Other things that will take them out are, salt, parasiticides and certain other meds. Had you run any of these?