Calci-Sand isn't a good primary substrate. It's expensive, smells when wet, and apparently hardens into a rock after it dries. Playsand or shredded coconut husk would be much cheaper and would work better. You can offer some Calci-Sand in a small dish for them to eat for extra calcium, though.
What's "good" as far as the temperature and humidity? They need at least 70% humidity and heat between 80-90 degrees. Also, you don't need a sponge unless the dish is too deep for your hermit crabs. They don't really help with humidity unless they're huge, and they grow bacteria. (Remember how the news media was saying how unsanitary kitchen sponges were a few years ago? Yeah.)
Is the Fruit Salad the all natural one from Zoo Med? If it is, that's fine, but if it's something else with tons of preservatives and chemicals in it, I'd switch to something else.
If you have extra shells in the tank, there's a possibility that the new hermit crab switched shells and buried down to molt and de-stress. Whenever I get a new one, they always seem to switch shells, gorge on food and water, and them bury down for somewhere between 1-2 months. Then they come back up sharp and newly molted.

I wouldn't worry about it too much unless you start to smell dead fish.
Really, all you should do is keep changing the food and water and let nature take it's course.