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A ten or twenty gallon tank is good for two to four small hermits.  A twenty to forty gallon tank is good for a dozen small or three to four large hermits. Hermit crabs are social animals and should have at least two other crabs with them.  The proper home for your crabs should be one that holds in humidity, but still lets in fresh air. A fish tank or reptile aquarium usually does nicely. You can even rinse out that old leaking tank from the attic and use it! Acrylic terrariums tend to work better, as they hold the humidity and heat more efficiently. You should buy a hygrometer (humidity gauge). These will help you to monitor and maintain a 75-85% relative humidity. Hermit crabs breathe through (hardened) gills, and can't breathe properly unless the air is humid enough. The ideal range is at least 75% relative humidity. Humidity lower than 70% will cause suffocation, which kills slowly over several weeks or months and is extremely painful. A great way to increase your tank's humidity naturally is to add natural moss to your tank. It increases the humidity, and hermit crabs eat it. Look for moss like Fluker's Reptile Moss at your local pet store. Sponges also work well, and can also be found at a pet store. But sponges get dirty easily and need to be changed out every two to three weeks. Hermit crabs are tropical animals and do best in warm temperatures. 75-85°F is the proper temperature range. Heat damage is irreversible, and temperatures that are too low slow down a crab's metabolism. An under-tank heater designed for hermit crabs, mounted on the back of the tank is a good way to keep your tank humid. An improper environment can cause your crab to become lethargic and inactive, lose limbs and could potentially cause death. Buy a thermometer to keep an eye on the tank's temperature. Substrate is the layer of material you place on the floor of the tank. Sugar-sized arrogate sand is the only sand you should use because other grades could cut the crab, and play sand may have harmful fills. Use dechlorinated salt water to wet the sand to a "sand-castle" consistency. You also can use compressed coconut fiber (Sold as Eco-Earth or Jungle Bedding). Expand the coco-fiber in the same salt water you would give your crabs to prevent mold/mildew. Substrates that crabs can not dig in, such as aquarium gravel (can not hold caves) or calcium sand (clumps and can have a foul odor) are not acceptable as a substrate. Your substrate should be at least 3-5 times the height of your largest crab, and should be a material the crabs can easily dig and build caves in to relieve stress, hide, and molt. Many crabs also like to bury and even molt in damp moss such as Fluker's Reptile Moss (not decorative or Spanish moss!) Dirty substrate can lead to mold which can be harmful to your hermit crabs. Change it every 6 months. However, every month you should do a brief check to see if there is any mold growing, or an ant or mite infestation. If you see either of these things, change the substrate immediately. It's a good idea to "spot clean" any dung and food the crabs have dragged from the food bowl or buried. You should only clean the substrate when your hermit crab is not molting (a crab who has burrowed underground and is going through a process to shed and regrow it's exoskeleton). Never move a molting crab.  If you want to go the extra mile, you can sterilize your sand substrate. Sand can be sterilized in the oven. Put the sand in a large roasting pan (one used only for this purpose!) and place it in the oven. Set the temperature to 250 degrees (F), (120 degrees Celsius) and let it be for about 2 hours. Once every two to three weeks, boil all the shells and dishes in your tank in a pan of dechlorinated salt water. Doing this will insure that that mold and bacteria will not grow and harm your crab. Allow the bowls and shells to cool before placing them back in the "crabitat". Hermit crabs love to climb! In fact, in the wild, they will climb large rocks exposed by low tide to search for food. choya logs or stumps are great for this. Choya is not toxic, and has holes in it for them to grab on to. You can lean it in a corner of your tank, just don't place it too high or your crabs will climb out. Lego's and hemp nets work well too. Hermit crabs are sometimes called "tree crabs" because they will climb trees to eat insects and vegetation. Do not, however, buy painted toys, as paint could be harmful to crabs if it is ingested.   Natural toys: Natural rocks and seashells that you pick up at the beach are great things to scatter around the "crabitat." Clam shells even make great food dishes. Just make sure to boil them in boiling water so that they become sterilized before you place them in the tank. Plastic toys: Plastic plants made for reptiles are great for crabs to climb and hide in, just remember to use the lid of your tank so the crabs can't climb out.  Make sure they aren't eating the plastic, and remove it right away if they are! Never use reptile "half pine logs", as pine is an irritant to the crabs and can be toxic. Hermit crabs, like most animals, want a place to feel secure and hide when they feel threatened. You can use a half coconut shell that they sell at pet stores for small crabs, or use broken pots, large shells, etc. Just make sure that the crab won't get stuck and, preferably, is able to dig out if it does. Live plants can be a great addition to any tank. In particular, plants like bamboo (make sure it's real bamboo and not Dracaena Sanderiana, which is sold as "lucky bamboo"), Venus fly traps bromeliads (air plants) and spider plants are among the safe plants. Be forewarned--your hermit crabs may snack on them, so there's no guarantee the plants will have time to grow. All species of hermit crab need to have access to both freshwater and saltwater. You will need to provide two water dishes for the crabs. Hermit crabs need to balance the salinity of the water in their shells; the dishes should be at least deep enough that your crab can get water in its shell (C. Perlatus, A.K.A. Strawberry hermit crabs, need to be able to submerge themselves), but make sure they are able to get out. A good way to do this is to provide them with a sloping water area, where one part is shallow and easy to climb out of, while the other is deep and they can submerge themselves in the water. Line the area with rocks or something they can grip on. Plastic is too slippery and they will have trouble climbing the slope.  If you have large and small crabs together, you can put small rocks or a small natural sponge in the water dishes so that it is deep enough so large crabs can let water in its shell, but small crabs will not become trapped in the water dish and drown. You can buy aquarium salt for marine fish (not freshwater fish) from most pet stores and a little goes a long way. Never use salt intended for human consumption because the anti-caking agents can be harmful. Most hermit crab brand salts are also table salts. Pre-mixed "hermit crab water" does not have the correct salinity. Use a brand like Instant Ocean, Oceanic, etc. The chlorine, chloramines and heavy metals in most tap water can kill hermit crabs by causing the gills to blister (causing eventual suffocation). Aging the water will remove the chlorine, but not the chloramines, so dechlorinator is a must if you use tap water. Zoomed water conditioner is a good brand to use. If you don't want to dechlorinate tap water, you can use spring water instead. However, make sure nothing has been added to the water. For example, Dasani water contains magnesium sulfate "for taste", and that is bad for the crabs.

Summary:
Make sure you get the right size of tank. Make sure your hermit's home is the right humidity. Make sure your tank is the right temperature. Get substrate. Keep the substrate clean. Get climbing-toys. Provide your hermit crabs with a hiding place. Add some live plants to your tank. Provide your hermies with water. Make sure the water is dechlorinated.