Then take all the shells and feeding dishes and boil them for about 15 minutes. While that is going on, give your crab a good bath. Submerge him in water for about 2 minutes and make sure you get all the air bubbles out of his shell. It won't hurt the crab, in fact, it is a good practice to do this once a week. Use dechorinated water as the crab's drinking water, too.
Chlorine is very bad for hermit crabs.
While you're at the pet store, get a lid to put on the tank. A regular glass or screen lid will work just fine. It will keep the gnats and flies out of the tank. Stay away from Wardley's crab food. Grind up the pellets into a powder.