Osmotic regulation in organisms can be a bit complicated, but the basic idea is that hormones regulate how much salt is kept int the body by the organs and how much is released. Salt water snails release more salt and fw
snails keep more. Organisms that can switch from fw
have hormones capable to regulating the change internally.
Like the others said, the salt used in qt is NOT the same as marine salt. Most fw
snails do not tolerate qt salt very well because they do not have the proper hormonal balance to get it out of their systems.