Some digital hygrometers may permit you to set an alarm if the humidity drops below 70%. You can find hygrometers at pet supply stores or general retailers.
If you want to trap in even more moisture, use a glass or hard plastic top instead of wire mesh. Or, stretch plastic wrap over the top of the wire mesh. Use a solid top that is designed for the aquarium. It will provide the necessary air circulation while also trapping in moisture. Despite their name, hermit crabs don’t like to live alone—they’ll be happier and healthier if you keep them in groups of 2 or more.
Chlorine is harmful to hermit crabs, so you must always use dechlorinated water. You can get dechlorinating drops at pet supply stores—follow the package instructions for dosing and use instructions.
Ideal indoor humidity for humans is 40-50%, and indoor humidity over 60% can lead to mold growth. So don’t try to crank up the humidity in the room too high.
Evaporation from the bowls will also help to maintain higher humidity levels in the enclosure. For average-sized hermit crabs, bowls with 1. 25 in (3. 2 cm) sides that will hold 1 in (2. 5 cm) of water are usually ideal. Smaller “hermies” may need smaller bowls that hold 0. 5 in (1. 3 cm) of water.
Get the salt additive intended specifically for this purpose at a pet supply store. Follow the package instructions for use. The hermit crabs will soak in both bowls, absorb needed sodium from the saltwater, and drink the freshwater.
The more hermit crabs you have, the more often you’ll have to replenish the water supplies.
If you ever spot a scummy film forming on the bowl or water, or if the bowls have become heavily soiled with droppings or food debris, clean them immediately.
You can buy natural sponges at pet retailers or online. Ideally, it’s best to have 5-7 sponges so you can switch them out daily.
The water in the sponge will slowly evaporate into the enclosure, increasing the humidity. You can place the sponge anywhere you like in the hermit crab enclosure.
If you notice that the sponge dries out more quickly, you may need to replace it twice per day.
It’s important to use dechlorinated water for this process as well. If you don’t sterilize the sponges regularly, they’ll become great breeding grounds for potentially harmful bacteria. The same holds true for kitchen sponges, by the way!
Your substrate should be deep enough that your largest hermit crab can bury itself fully in it, if it chooses. This often requires 2 in (5. 1 cm) or so of substrate. You can add more if you wish, but only if you don’t have smaller “hermies” who might bury themselves and not be able to dig back out. Coconut fiber is a great choice for hermit crab enclosure substrate because it holds moisture well, resists rapid mold growth, and can be safely nibbled on by the crabs.
Give the sand a heavy mist of dechlorinated water after you place it in the enclosure. You can use sand alone as the substrate if you wish, but coconut fiber holds moisture better.
If you can’t tell if the substrate is dry or not, stick your finger down into it to feel for moisture. Wash your hands thoroughly before and after doing this, though.
The more often you pick through the substrate to remove debris, the longer it will last.
New coconut fiber should be used at this point. You can, however, sterilize and reuse the sand if you wish. To sterilize sand, place it in the bottom of a bucket, then slowly overfill the bucket with dechlorinated water until the water doesn’t look cloudy. Pour off the remaining water, add the sand to baking sheets, and bake it in the oven at 300 °F (149 °C) for 1 hour. [19] X Research source
