When you obtain new shells for your hermit crab, it is very important for you to make sure the shells are safe and sanitary. You can do this by boiling your shells. First, use a brush to remove any sand or debris from in the inside and outside of your shell. Then, submerge your shells in boiling distilled water, and boil them for 15-20 minutes. After your shells have been boiled, they must be moistened with ocean water, or salt water. (Purchasing a saltwater solution from a pet store is the most effective option.) As you wait for your crab to select a new shell, you will want to periodically re-moisten the insides of your shells. This will draw you hermit crab toward the shells, and make the shells more attractive to him. Avoid tap water. Most tap water contains trace amounts of copper or other heavy metals. Hermit crabs are extremely sensitive to these metals, and they can hurt your crab. Once your shells have been properly prepared, place them where your crab can see them. Position them out in the open, on the ground level (substrate) of your crab’s habitat, so that your crab will encounter them daily.  Once a week (or every other week) you may want to remove the shells, wait a few hours, and then put them back. If your crab has become accustomed to the shells, he may not really recognize them anymore. Removing and reintroducing them may encourage your crab to investigate, and possibly even change shells.  This is also a good time to re-moisten your shells with saltwater.

Summary:
Boil new shells. Moisten shells with saltwater. Place shells where the crab can see them.