If your tank conditions are optimal and you have followed all the instructions on providing proper climate and care to your hermit crab, then most likely you did nothing wrong.
Land Hermit Crabs endure a great amount of stress before reaching the pet store. The harvesting and shipping of hermit crabs is a very inhumane process and the crabs suffer because of it. They then arrive in pet stores, who in most cases, dont know how to properly care for them. They arrived stressed out, dehydrated and hungry. Then you purchase them and take them home. Often it takes a period of time for the stress and dehydration to catch up with them but it does and the hermit crab dies. Sometimes its a week, or a month but it could take longer. Gill damage from dry air (lack of humidity) can be a slow painful death for a hermit crab.
There are some ways to give your new pet hermit crab the best possible chance at survival. When you purchase new crabs, take note of the store conditions. If there are no gauges, do your best to guess the humidity. When you bring home a new hermit crab, place it in an isolation tank with the same humidity as the store. Use a tiny amount of moss if needed to create the proper humidity. The temperature should be at 72F and remain there. Place a very small amount of sand in the tank, not enough for the crabs to burrow in. Leave the crabs alone except to change food and water. This will allow the crab to relax, destress and get enough to eat and drink. It is very important that your crabs eat well during this time. Food fuels metabolism and this is how they will adjust to the tank settings as you change them. Once you have a consist starting humidity at or near that of the store, hold it there for four days. In four days, add some more moss to bring the humidity up 5%. Continue to increase in this manner until your ISO tank is the same humidity as your main tank. Between the 3rd and 4th week, add more sand so that the crabs can dig down if they want. If you buy more than one crab, it is okay to place them in the same isolation tank together. Its a good idea to bathe the hermit crabs before placing them in your main tank. Use plain dechlorinated water. This bath is not required but it seems to be helpful. There is no evidence that stresscoat is actually beneficial to hermit crabs, or harmful for that matter. It is up to you if you want to use it. This is a good way to check for parasites (mites) before placing the new crabs in the main tank. If the hermit crabs have mites (parasites) you will should keep them quarantined until the mites are gone.
This information is based on the exclusive research of Susan Latell, All Rights Reserved. View the her full synopsis here: PPS: Minimizing the impact by Sue Latell of Coenobita.org