Hermit crabs bathe themselves naturally on their own, but there are a few reasons you should give your hermit crab a bath:
1) When bringing a new hermit crab home. Before introducing a new crab to your clan of clawed family members, you may want to give your crab a quick bath (this eliminates all contaminents from its shell and body).
2) If your crab has mites. Mites aren't a fun bug for any pet to have, but they're certainly not healthy for hermit crabs. Luckily- they don't have gills, so they can easily be eliminated with a bath.
Giving your hermit crab a bath is quick and easy- but that doesn't mean it should be done often. ONLY bathe your crab when they are in the condition of either of the two reasons listed above. If they do need a bath here is what you need to do:
Fill a bowl with enough water to cover your hermit crab's shell completely. Make sure the water is dechlorinated (free of chlorine) and room temperature.
Place your crab upside down, into the water. Make sure the opening of their shell is facing upward. This will allow water to fully run back into your crabs shell.
Remove your crab from the water. Most crabs will naturally stretch up and out of their shell to reach the air. When they do this, grab your crab and remove it from the water. If it doesn't reach up- grab it within 4 seconds of placing it in the water. Make sure your crab is facing down so the water can run out of their shell.
Check the water. If there are mites in the water, dip your hermit crab once again following the technique above. Continue this process until you see no additional mites being left in the water. The mites will naturally float out of the hermit crabs shell once it's submerged in water. Once mites quit appearing you can quit bathing your crab. If your crab doesn't have mites- one dip in the water works fine.
Place your crab in it's nice warm habitat and let it readjust to it's new clean shell.