Hermit crabs do not need huge elaborate homes, but the appropriate temperature and humidity is absolutely vital to their health. Remember that land hermit crabs come from warm tropical climates and therefore need a warm, humid environment to survive. See the end of the article for some online retailers specializing in hermit crab supplies.
Glass or plastic tanks can be used for hermit crabs.
A 10-gallon aquarium with a lid (a sliding glass one will work well to contain humidity) is a good choice. Plastic tanks with lids are now readily available in a variety of sizes and the larger ones of these can be used the mesh lids (don't retain humidity well) make them less optimal for long term housing. The small plastic homes with accessories sold as hermit crab kits are too small. These plastic cages make excellent temporary homes or isolation tanks, though. As for minimum size, there are varied opinions, but a general rule is that the larger the cage or tank, the better. I'd recommend the equivalent of a 10-gallon tank as a good size, though, as it allows room for the cage furnishings needed and offers enough space for a few crabs. Despite their name, hermits are quite social and are best kept in groups.
Sand is generally the substrate of choice for hermit crabs, although there are a couple of other options.
Sand is nice because the crabs like to burrow down into their substrate and sand provides a good outlet for this natural behavior. Playground sand (found at home improvement stores) works well and is inexpensive. You may want to rinse, dry, and bake the sand (at 300 F) to sterilize it, and it can be re-washed and used again.
Aquarium sand is fine as well. Calcium-based sands are nice and come in a wide array of colors but are expensive.
Other options include fiber bedding made for reptiles such as the coconut fiber-based bedding called Forest Bedding. The fiber is ground quite find so it is nearly like soil, so it is good for burrowing. Crushed coral is also a nice choice, but you may want to provide an area with Forest Bedding or sand as your hermit crabs may prefer these substrates for molting. Avoid the use of gravel or wood shavings.
Hermit crabs are happiest kept at 72-80 F (22-27 C). If the temperature drops below 72 F on a regular basis, the crabs will likely become weak, stressed, and ill. Unless living in a tropical climate, most owners will need to use a heater at least part of the time to keep the crab tank at optimal temperatures. Undertank heaters, lights, or a combination of both can be used to maintain appropriate temperatures.
Undertank heaters (UTH) can be placed under one end of the tank in order to provide a warm side and a cooler side. These will generally raise the temperature a few degrees above room temperature. For ideal temperature control, these can be combined with a thermostat to maintain a given temperature, or you can put them on a timer to come on and off to maintain temperatures.
Invest in a good thermometer for inside the tank, and monitor the temperature near the substrate. If the heater isn't heating the tank up enough, try removing some of the substrate over the heater - the thinner substrate will increase the heat in the tank. If the tank is getting too warm, you can increase the depth of the substrate. Some experimentation may be necessary to get good, stable temperatures. Make sure that there is a temperature gradient in the tank, so the crabs have a choice of what temperature they want to be at.
Lights of various types (see below) can also provide heat for the tank; some experimentation with lighting may also be necessary to find the combination of lights and UTH that works best for your tank.
In the past, it was thought that hermit crabs were nocturnal and providing lights could be stressful for the crabs.
However, low wattage and/or special night light bulbs are a good option and many crab owners have found their crabs became more active with lighting and even bask near the lights when lighting was added to the tank. Be sure to provide a light-dark cycle (12 hours light and 12 hours dark seems to work well). This means day glow or fluorescent bulbs should only be off at night (at night, special night time bulbs can be used if desired). Using a desk lamp to heat the tank, or high wattage reptile bulbs, may overheat the tank and be too drying so these are not recommended.
The easiest way to add lighting is to use a reptile heating/lighting hood over the tank. Hoods can be found with two ceramic receptacles for incandescent bulbs; a day glow bulb can be put in one side, and a night glow bulb in the other. It is best to start with 15-watt bulbs and go to higher wattages only if necessary, especially with a 10-gallon tank. If necessary, wood slats can be used to raise the hood a bit above the glass (if the glass gets too hot). Some hoods have a third receptacle for a fluorescent bulb, and some owners have reported good results with a Reptisun 2.0 fluorescent. Reptile heat hoods are best used on glass top tanks (or screen, but screen tops make humidity regulation difficult) as the lights will be quite warm and could melt plastic.
A combination of lighting and undertank heat can be used to heat the tank.
Along with the proper temperature, adequate humidity in the tank is vitally important to hermit crabs. Since crabs "breathe" via gills, the proper exchange of oxygen by the crabs depends on the humidity in the air, so if the tank air is too dry the crabs will essentially suffocate. They need a relative humidity of around 70-80% and since this is so important to the crabs it is worth investing in a humidity meter (hygrometer) so you can make sure the crabs will be okay. You should be able to find one in the reptile section of the pet store. Excess humidity is not desirable either as it will cause condensation as well as encouraging the growth of bacteria and fungus in the tank.
The water dish you provide in the tank will likely be sufficient for creating the proper humidity, as long as the tank is enclosed (solid sides and top). If you need to increase the humidity level, try a fairly large chunk of natural sea sponge in a dish of water (remember to always use dechlorinated water). The sponge can hold lots of water and has lots of surface for evaporation to boost the humidity. Have a couple of sponges on hand so you can swap and clean them frequently (soak them in very hot dechorinated water or a sea salt/water mix, then allow them to dry completely as they are a good medium for bacterial growth). If a mesh or vented lid is making humidity control difficult, the lid can be modified by covering most of the top with plastic wrap or clear packing tape.
There are three necessities for furnishing the cage: stuff to climb on, a water bowl, and food dish.
Land hermit crabs love to climb and this is a good way to provide some exercise. Choya (or cholla) wood is ideal and can be arranged to allow climbing. Pieces of coral, driftwood, and other types of wood can be used - the reptile section of the pet store is a good place to look for a variety or check the online hermit crab retailers listed below. Look in the aquarium, reptile, and small animals sections of your pet store to find a variety of interesting climbing, basking, and hiding structures that are easy to clean and could be used for hermit crabs. Artificial plants are also a great addition to the crab tank. You can periodically change things around or add different items to provide some variety and interest for the crabs. Some enterprising individuals even use Lego to build climbing structures for their crabs.
Dishes - Water and Food
For food dishes, you will want something shallow, sturdy, and easy to clean. Flattened heavy plastic dishes made to look like rocks can be found in the reptile section, or you can use shallow ceramic dishes made for small animals. Some people also use natural sea shells (the flatter half shells) for feeding.
Since all species of hermit crabs should have access to both fresh and salt water you will need two water dishes. They should be big and deep enough to let the crabs get into them if they wish to soak (especially the salt water dish), but easy to get out of and not so deep that drowning is a risk (strawberry hermit crabs should be given a salt pool deep enough to fully submerge themselves in, but for most species it does not need to be that deep). With deeper dishes, smooth river stones or pieces of coral can be used as ramps or steps for the crabs to get out of the water. ZooMed makes a nice water bowl (Hermit Crab Ramp Bowl) that is attractive and is good for hermit crabs due to the stepped edges for easy access in and out, and their Repti Rock dishes work well too. You should also place natural sea sponges in the water dishes; some crabs will press on these to get water to drink and they help regulate the humidity.
All water given to the hermit crabs or used in the tank should be dechlorinated (drops available at pet stores). Salt water should be prepared using a marine aquarium salt such as instant ocean (mix as for saltwater tanks), not the salt made for freshwater tanks and never table salt.