There is lots to learn about hermit crabs, so bear with me!
Hermit crabs are a social creature. They live in huge colonies in the wild and will get lonely alone. For them to reach their maximum potential, I recommend having at least two. That is one of the first mistakes people usually make when getting hermit crabs. They need a friend! I have 7 currently in a 55 gallon tank.
For one hermit crab, you should always have at least a 10 gallon tank. They are used to having a beach to roam on, so the more space, the better.
For the tank, one of the best things to get would be a plexiglass lid to help you keep humidity. You can do a mix of Quickrite playsand (or just any playsand but this one is commonly used) and Eco Earth (coconut fiber) or straight sand or Eco Earth. I use straight Eco Earth because it makes keeping humidity a breeze. You can buy it in big blocks at almost any petstore that has reptile stuff. For two small-med crabs I would do about 6 inches of substrate. Deeper is better, these little guys love to dig! Plus, they dig under the surface to molt. The substrate should be sand castle consistency if it is sand and damp if it is Eco earth/coco fiber.
You need a temp/humidity gauge. You can look at walmart for a digital one that is pretty cheep (I forget the name, someone else remember?) or go to petco or petsmart for one. Your ideal temp is 80 F and humidity 80%. You will need a covered lid (not a screen lid) to keep in the humidity. You can use spaghnum moss from the petstore (it is not treated) to help keep humidity. You will also most likely need a spray bottle for that. For heat, you can use a dome light and a night glo bulb (so that you can keep it on at all times if you need to and not have to switch from night glo to day glo reptile bulbs) to keep heat. You can also use an under the tank heater if you place it on the SIDE of the aquarium. We usually recommend the next size up if you will be getting one of those.
Hermit crabs have specific nutritional needs. They need proteins like shrimp or other meats. They need fruits, vegetables, and a source of calcium. You can use cuttlebone for a source of calcium.
They also need dishes for water that they can submerge themselves in. You need dechlorinated fresh water and marine grade salt water. Most people use Instant Ocean. The little bottles of "Hermit Crab Glub" and such do not have the full set of minerals like Instant Ocean and other marine grade salt water mixes have. You need to make sure that you are mixing the salt with water that the water is dechlorinated.
http://www.hermitcrabpatch.com/Hermit-C ... -a/148.htm
This is good example of a basic setup. If you look towards the bottom on the far left there are other care guides.
Hermit crabs also need hiding places,lots of stuff to climb on, and shells to change into (not painted ones, they are toxic!).
Hermit crabs are much more complicated than most people realize, and they even require much more care than a dog or cat, in my opinion, though they do not want or need as much human attention as dogs and cats do. I would not get one for a small child without making him or her completely aware of their care, and it is quite complicated. Hermit crabs can also take months to molt, but in that time you still have to maintain the tank. It might be boring for a little kid, but it can also be very enriching if they spend time learning about their care, behavior, and such. They are fascinating little creatures, and I encourage you to keep looking around and reading extensively on these little guys! Here and The Hermit Crab Patch are great places. These are NOT beginner pets, and again, they hide all day because they are nocturnal and people loose interest very quickly. Good luck!
