The changing of the substrate could be part of the problem as well. Taking out all of that old sand more than likely stirred a bunch of gunk up as well and released it into the water column. I suggest bringing the salinity up to 1.025/1.026 slowly through some water changes and make sure no more tap water is used, only use good clean RO/DI water. Once you get the salinity up, then try a snail or two and see if they do better.
