Sanils are generally very hardy animals as they come from the shallow water areas where water temperatures vary a great deal from day to night and the salinity of water changes rapidly from rain and evaporation. I have many snails in my tank and just flow to adjust temp before adding to main tank. Copper is the primary killer of inverts, you may not have a penny in the tank but if your water source comes through copper pipes as many homes have then you may be adding copper to the water and not knowing. I would check the water for copper content, high levels of iron can also be the cause as can aluminum in the water. Storing water in a galvinized can will be lethal as well. While copper is the most likely, other metals on the chart that are in close proxsimety to copper will create an issue in high amounts. also there is a practice of adding Calcium Cholride that is used to desalt roads in the northeast, this adds a lot of cholrides to the water and will kill inverts first. These are just a few things that can go wrong on the chemistry front, then there is the chemicals used to clean the glass, high ammonia in the water or high nitrites. I know this isn't very helpful for your situation, but does give you a few places to check out and look at. Gosh, I do wish you good luck as snails are so necessary in this hobby.