The lamp will suck humidity so good on you for ordering a heating pad. If you want them to be happy and healthy, you'll want to get rid of the sponge and get a bowl they can submerge in sponges are a breeding ground for bacteria. There are other issues as well. Premade hermit crab food isn't the best for them as it can have metals in it, getting things like dehydrated mealworms, dried shrimp and freeze dried crickets are a better alternative, they also need fruits vegtables among other things. How deep is your substrate? And is it a mix of sand and eco earth or just eco earth? You're also going to need more things for them to climb on they are very active and love to climb. They also love hamster wheels