snails can be strange sometimes and they are very sensitive to water conditions. do you have other invertebrates in the tank things like shrimp? i do know that snails are more sensitive to changes in SG levels so looking into that and maintaining 1.023 - 1.025 more consistently with fresh water top offs between water changes may help as well. the only other test you may want to consider is a copper test in the water, im not sure where your setup came from but if its not all brand new then you may be dealing with trace copper levels. all of that said i have a 90g tank with pristine water conditions full of corals and anemones shrimp and crabs and i can not keep snails in it they die within a week every time with no explanation. Really all you can do is gather as much information as you can on conditions for your snails, make sure you drip acclimate i actually drip acclimate and then re float the bag to get temps back to matching, and be a meticulous as possible with water quality and try again.... hope that helps and that you have better luck with snails than I