Water changes can bring on a moult, they'll then hide for a couple of days while the new carapace hardens.
Snails just grow their shells.
If the water is that soft now it may be worth using cuttlefish "bone" to give a calcium boost. The cheapest way is to buy one for budgie beaks and cut a piece off, pop it in boiling water for a couple of minutes and then drop in the tank, they'll graze on it. Put the rest in the fish cupboard and break off bits as they disappear in the tank. It should last years.
