Water is very indispensable. The most common cause of death among hermit crabs is dehydration. To provide the necessary moisture shallow pond should always have fresh clean water. It is also important baths only at least two or three times a week, particularly when the weather is warm and humid.
Clorine in their drinking water, temperature to hot, cold or too much fluctuation, improper humidity level, mite infestations, severe stress, poor nutrition, aggression from larger crabs i. e. , fighting over shells.
If they have discolorations, they may have died from a fungal infection. Something in their environment may have changed, such as water temperature or chlorine. Lastly, it may have had mites or flies in an unclean environment making them sick.