HCA: Hermit Crab Association

Hi!
New crab owner here. I'll try to be as thorough as possible. Okay, here we go.

I let the kids get hermit crabs on vacation. Came home and set up the crabitats. I followed the suggested requirements to the best of my ability; 6 inches of mixed play sand an eco earth, they have dechlorinated fresh water and salt water available, fresh fruit/vegetables, protein, a hiding place, little saucer, and plastic plants to climb/play in. Temperature is good and humidity is good.

After the tank was set up, I knew each tank needs additional crabs so they aren't alone. I bought PetSmart's last hermit crab a week ago. The newest addition is bigger than the first, it's shell opening is 1.1/4" (is that big?) Anyway, Biggie dug into the sand on day 2. It's been underground for about 5 days now. Today is a water change day (I've been changing every 2 days) and when I removed the saltwater bowl I saw Biggie's shell! Where it is now is the opposite side of where it dug down. The crab wasn't there two days ago when we changed their water last. What should I do? I don't know if it's molting or destressing. Should I gently cover the crab with sand and move the water bowl? Or put something over the crab and put the bowl on top of that? If I should cover the crab with something, what do you suggest that is? I'm kind of at a loss. I'm worried about disturbing it and the ramifications.

Welcome to crabbing and the site!
Don't put any sand over top of the crab - he has a little 'cave' where he has oxygen, and if you pile sand on him he could suffocate.
Crabs often move once they are underground, so him being on the opposite side of the tank is very normal.
What are you using for water bowls? Many of us use plastic Rubbermaid type food containers, so that they can be doubled up - then when it's time for water changes, the bottom one stays in the tank and just the top one is lifted up. Because, you guessed it, burying under a water dish is a thing with crabbies LOL. Do you have space to relocate the water dish? Was it dug into the substrate, or just sitting on the top?

Thank you for the reply! Yes, they're Rubbermaid dishes. I didn't think about doubling them up before. I do have space to relocate the bowl. Should I relocate it or place a second bowl there and leave it?

If you can very carefully put a second bowl there, and not collapse the cave, I would tend to do that.

Just a thought about doubling up for in the future: my water dishes are actually old feta cheese containers. I have them side-by-side in a plastic container from the dollar store. So it serves the same purpose as doubling up with the same container, but because they don't nest, it's super easy to pull them out and they never stick together.