Be careful with the shells, you want to bring ph gh and kh up slowly. While ph is important, you need to know the hardness of the water as well, which would be the gh (general hardness) and kh (I'm still not positive what it stands for) ... Gh is basically the mineral content that your water has, and kh has something to do with the buffering of the water.
As for lethargic snails, make sure they've got food. Did you just get them? Did you change the oh and hardness too quickly? The sudden swing could hurt them.