My mystery snails are sick after other snails died.?

Mystery Snails are wonderful creatures. I own at least 8 of them. You have to change their water once a week due to them leaving waste quite frequently. Aquarium Snails diet consists of Algae wafers and Fresh Leaf Spinach. Put your snails in temporary container with water and clean their home and put them back in and feed them afterward. Consider it their treat for being good little troopers. I should know. Snails are delicate creatures and if you take care of them well, they will even crawl around and respond to you and produce young. Now that is cool! Hope this little advice helps..