Did you cut any plants or add new ones? I had a tank wipe out after adding anubias nana petites and didn't know better about plants then. Now I soak them and rinse them for a day or so. It's possible in my case, the seller had used something to clean the plants before selling them. I think shrimp could possibly be a tad more sensitive. Depending on what kind of snails you have, ones with a foot might close up until whatever irritant is removed.