Thank you for your question. Take your hermit crabs out of the tank and put them in the bowl or small container. Then take the bottled distilled water and put it in the microwave with the cap off for 30 seconds. The water is should be lukewarm. Then put the water in the small container or bowl that the crabs are in and let them soak in the water for a few minutes. You may need to tip them upside down so the dead mites come right out. After their bath, put them in a towel to dry. Do not put them in the tank after the bath or they will get the mites back.
To clean the tank you first need to take all the decorations out of their tank dispose of them. Any cleanable items, like food and water dishes should be washed thoroughly. All the substrate in your tank will need to be replaced as well. Put the tank in a sink and fill it with water and then dump it out and do it again till all the mites are gone. Wipe out the tank with a towel and set it aside and take all your new decorations and food bowls and wash them off with water and towel dry. You can then add the crabs back to the tank.
You may want to only feed your crabs dry food for awhile because strong smelling foods or fresh foods may bring the mites back. Replace the food and water every day. Pay extra attention to keeping the bedding substrate clean by scooping out any feces or buried food.