Turbo spp. (turban shells (turbo grazers)
Good choice for reef aquariums, but some snails offered for sale, particularly those collected in Mexico, are subtyropical animals and do not survive well when housed in a normal reef temperatures.
Tegula funebralls - Margarita snail (margarite snail)
Come from the rocky intertidal of the Pacific coast of North America. Can tolerate only short periods of warm water in the summer in natrue. survive a few weeks or months in reef tanks then they die from being slowly cooked.
So If it were me I would go back to the lfs and get a credit for selling you snails for your marine tropical system that were not suitable. You always need to make sure you know where the animals come from and research before purchasing to make sure your system can support the animal.
Reflecting back on your fish dying on you, Ive been reading some articles and it almost seems like they have suffocated. not enough oxygen during the night hours, this can be caused by excessive bacteria, hydrogen sulfide from the substrait, lack of gas exchange at the surface, or, a contamination of the air environment around the tank, tobacco smoke, exhaust fumes from vehicles, or etc etc etc. just some thoughts. I really do believe your if fish were unmarked, they were dying from lack of oxygen.
good luck happy fishin.