Hermit crab habitats are much debated and are very very crucial to a hermit crab's lifespan. When taken care of properly, hermit crabs can live 10-30 years in captivity. They are a commitment, just like any other exotic pet. They usually die within days due to improper care.
Hermit Crabs ALWAYS require AT LEAST 70% humidity as they breathe through hardened gills. Without this they will suffocate and die :( Fear not! This is much easier than it sounds.This can be achieved with a misting bottle of NON-CHLORINATED (bottled, filtered or treated) water. Tap water is toxic to crabs due to the chlorine. Spritz a few times a day and monitor with a hygrogometer (cheap and easily found at any pet store). That's it!
Humidity can also be helped with the proper TANK. They need to be glass (or acrylic in a pinch) to hold in the humidity, preferably with a tight mesh or glass lid. Doing so will keep the environment hot and humid and allow less humidity to escape, which means less misting.
DO NOT EVER, EVER, USE ONE OF THESE:
Not only do they break easily, they don't hold in any humidity and your crab can get stuck in them, they don't have space for SAND! Sand is crucial for crabs. The amount of sand needs to be at least twice as tall as the height of the hermit crab, as they will burrow underneath often for extended periods of time to molt. COLORFUL FISH TANK ROCKS DO NOT WORK FOR THIS PURPOSE. They won't hold a burrow together and can scratch your crab's delicate skin. They can die from not molting as well. Playsand is the best option.
Make sure your tank is tall enough for sand and climbing toys. A squat tank for a spider will not work. They can climb out of ANYTHING (even glass) so a lid is important.
The final requirements: Food, water, and heat.
Crabs must have two pools that are constantly full of water: One salt water (bottled salt water for crabs can be found at walmart) and one fresh, non-chlorinated water pool. Tap water must be dechlorinated or treated to be fit for use! Each must be shallow, as they can drown, but need to be able to go partially to fully underwater. These (and only if needed a natural, damp sea sponge) can also aide in humidity, and are crucial.
Hermit crabs love most crab food, just make sure it is not loaded with preservatives, which are also toxic to crabs. We recommend Hermie's Kitchen on Etsy.com or the Hermit Crab Patch for cheap and safe food. Hermit crabs also enjoy fruits, vegetables and seafood. Feel free to check out the hermit crab association website for ways to supplement their diet!
Crabs need 70-80 degrees Fahrenheit, and light during the day. This can be achieved with an under tank heater and/or heat lamp. (or at the very very least, a very warm room with a light on! Such as a greenhouse, sun-room, or screened porch.) Under tank heaters must be placed on the side or back of the tank to heat the air, not the substrate on the bottom.
Following these parameters will ensure a healthy, happy crab.