HCA: Hermit Crab Association

I posted this in the new member section but realized I should have posted in this section. I deleted the pre-made questions because I think I already answered them all in my post. So let me know if I missed something.

Hello everyone!
I'm new to this group and a new hermit crab owner. I've been doing a lot of research and reading the posts on here. But I need to make my own because I'm not sure what to do about my crabs. Very worried about them.
So I got 2 small crabs about 2 weeks ago(purple pinchers I think). I got them a 10 gallon tank and will try to get a bigger one soon. Filled their tank with about 2-3 inches of substrate(Coconut fiber and sand). I gave them a hiding area and a climbing stick. 2 water bowls- one salt water and one fresh(using ZooMed purifier and salt treatment for tap water). Then originally got ZooMed pellets for food. Had it out one day and then read they were not good, so changed to peanuts, spinach, apples, mangos, etc. I bought a temperature and humidity reader and it's been staying around 78° and 70-90% humidity. The tank has a mesh top but I put aluminum foil on top with a hole for air. I bought 2 of those heater pads and put them on the sides of the tank. I put about 6 different shells in there. Different sizes and shapes. I put moss in there as well.
So the first few days they hid some but also were drinking and eating. I noticed the peanuts were moved around the tank. They also changed shells.(one changed many times before he found a good one). I ordered a mesh climbing thing to put on the back of the tank. And they were climbing it right away!
Then I decided to add some more substrate because I read they need more. So went to the store to get some more and also get some more climbing stuff(cholla wood and another grass mesh climbing thing because the first one was in the substrate and molded)
While at the store. I decided to take a peek into their hermit crab tank. I noticed this one big guy. He was in this tiny shell and was practically hanging out. He wasn't moving much and the pet store guy acted like "oh well". So I decided to rescue him.
Came home and put more sand and coco fiber in..it's about 5-6in on one side going down to about 1-2 inches on the other side where their water bowls are. I found a grass mesh hammock that fit much better than the first one and put it up. I added a bunch of cholla wood too. Put the new crab in and gave him a bunch of new shells, hoping he'd feel much better in a bigger shell. He was literally hanging out of his shell. His legs looked weird too. Like out all sideways. I wasn't sure if maybe it was just cuz he couldn't fit them in the shell or if it was something else. He stayed in the corner for hours. Not moving at all. The other crabs were feeling him out and I was worried something might happen so I moved him over to the coconut house so he could hide in there. That night I happened to catch him climbing around. Next morning woke up and he'd dug a hole about an inch deep. I could still see him inside. And he never moved.
I don't know if it's because I added more substrate or the new crab because the other crabs have burrowed down and haven't come up in days. Water not touched. Food not touched. Sand still smooth and untouched. I keep checking on the big guy with a flashlight and I see him moving when he sees the light. Just small movements. I also hear squeeking and chirping under the sand so I know at least one of the other 2 are alive but they have not come up in days. Then a few days ago, I noticed the big one wasn't responding to the light anymore. I waited 2 days and kept smelling to see if I smelled death but nothing. Then I decided to touch his shell and I saw very small movement. So he's alive. Not sure if he's molting or what. But the other crabs will not come up now. Are they molting too? Are they scared of the big one? Should I move the big one to a smaller isolated tank? I'm scared if I do nothing, the other 2 will die under the sand. But I'm scared if I try and move the big one, he'll die.
Anyway. Sorry for the novel. I wanted to make sure I put all the details.
Help please!
Ps- how can I post pictures? When I click the "add pictures" image it just puts the words "Img/img" but no way of letting me add photos

You need to use a photo sharing site, or hte app TapaTalk to post photos.

Crabs will bury and stay under for days, weeks, or months. It is normal. Your substrate needs to be at least 2 to 3 times as deep as your largest crab, through the whole tank. They need deep substrate so they don't come across each other when digging around under ground. As a general rule, don't dig up your crabs or go looking for them unless there is an emergency like a bacterial bloom or a flood.

There is a care sheet that outlines how many crabs of what size are appropriate in what size of tank.

Crabs need a variety of foods, including protein and calcium daily. There are care sheets about nutrition and safe foods.

The water conditioner and salt you are using are unfortunately not going to provide everything crabs need long term. There is a care sheet that explains all about water.

Thank you!
Yes I read that they can burrow like that for days but since it happened right when I got the new one, and the new one seemed sick or something, I was worried there might be another cause. Do you think I should move the big guy who's not moving? Into an Iso tank? Yes I got some calcium blocks that I crushed up and put on their food. Apples, mangos, spinach, nuts, etc. But they're not eating or drinking anymore at all now...also I will put the substrate across the whole tank instead of just on one side.
Thank you for replying!

I would wait a bit more to see what the big guy is going to do.
The ones underground don't need to eat or drink while they are under - they will wait until they come back up.

Ok ill keep yall posted!
Thanks so much!

Yeah i second everything curly said. Sub def needs to go across the whole tank. Also I think it would be fine to keep offering the crabs pellets until you can get a better quality food. You can make it yourself or purchase from online sellers. Very high quality food mixes are available from online sellers quite cheaply. Etsy is a great place to find high quality crab food. It's very common for crabs to dig down for long periods of time especially when they are new to your tank. My crabitat goes through phases where they're all up and it's like a crab party and then they all seem to go down at the same time and I will hardly see a crab for a month or more. Best to leave them alone, keep putting out food, and try not to worry about them. Keeping crabs requires a lot of zen.