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Make a plastic cave. Make a clay hiding place. Have multiple hiding places. Test and treat your rocks. Care for hideout toys.
Use PVC pipe to make a cave for your crab. Cut the PVC pipe in half and bury it in the substrate to make cave. You can bury the pipe horizontally or vertically.  You can also place the cut pipes side by side for a different look.  If the PVC is untreated, rinse it with clean water before you place it into the crab's tank. Make sure the cave is big enough for your crab to move in and out of it. Purchase a new, untreated terracotta flower plot as a hiding place. Place the flower pot on its side and dig it into the substrate so that it is secure.  You can also make your own pottery piece to put in the tank.  Only use natural clay without additives when making your own pottery pieces.  Do not paint, glaze, or glue any of the clay pieces.  These chemicals are harmful to your crab. Use sand paper to remove any rough edges from your pot and wash it with hot water to get rid of dust. Hermit crabs enjoy a variety of places to hide and play.  Incorporate different materials and shapes into your crab's habitat. An even mix of open space and hiding space is ideal. Your crab may prefer some toys to others so you can adjust the habitat based on your crab's personality and preferences.  Fake plants, vines, and rocks can also be used for hiding places. You could also attach rocks to a the outside of a clay pot or PVC pipe cave. If you use rocks, attach them using aquarium safe silicone only. If you collect rocks instead of purchasing them from the pet store, you need to make sure they are safe to place in the tank. Wash and scrub the rocks and pour vinegar on them. If the rocks bubble, they are not safe to use.  Place the other rocks in a bucket of water with 1-2 cups of bleach.  Allow the rocks to soak for 24 hours.  After 24 hours, rinse the rocks and soak them in a bucket of clean water for 48 hours.  During the 48 hour soak, change the clean water every 12 hours to make sure that the bleach is gone. Boil the rocks for 6-8 hours in a large pot of fresh water, rinse them, and allow them to cool before you use them. If you do not want to go through this process, you can purchase rocks from the pet store that have already been sanitized. Transfer your crab to a small plastic container with a little bit of sand and remove the toys. Wipe the toys using warm, dechlorinated water.  Some toys (i.e. drift wood and rocks) can be boiled for a few minutes to kill bacteria.  Other toys (i.e. plastic caves, plastic plants) should not be boiled, just wipe them down.  Allow the toys to dry completely before you place them back in the tank.  Never use tap water to clean the toys. Hermit crabs are sensitive to chlorine found in most tap water.  Also avoid using and soaps or chemicals because they are harmful for your crab. Toys should be cleaned every 2 weeks to prevent mold and bacteria from growing on them.