mix of play sand and coconut fiber 6" deep
2. Do you have gauges in the tank to measure temperature and humidity? If so, where are they located and what temperature and humidity do they usually read?
I usually have one but am waiting on another in the mail as they buried and broke the last one while I was out of town about a week ago. Usually it read 80-83 for temp and 80-84 for humidity.
3. Is a heat source used in the tank? If so, what?
a heat pad and a heat lamp
4. What types of water are available (fresh or salt) and how is the water treated (what brands of dechlorinator or salt mix and what ratio is used to mix it)?
Both fresh water and salt water, I dechlorinate with stress coat a few drops in a gallon of water.
5. What kinds of food do you feed and how often is it replaced?
I use hermit crab patch daily mix, wings and things protien mix, a fruit mix they sell, and their soil mix. replaced every other day.
6. How long have you had the crab and what species is it, if known?
I have had the four purple pinchers for two and a half years.
7. Has your crab molted, and how long ago did it happen?
I think they were molting right now, two seemed to have molted more recently maybe a month ago if not a bit more but two others hadn't molted in awhile including the one I know is injured.
8. What type of housing are the crabs kept in, what size is it and what kind of lid is on the housing?
21 gal long glass tank with a mesh lid that is sealed with press and seal except for a strip at the end for air flow.
9. How many crabs are in the tank and about how large are they?
four crabs, the largest with her shell takes up the palm of your hand
10. How many extra shells are usually kept in the tank, if any?
there are lots. right now I'm counting ten but they've also buried a couple that I've yet to find.
11. Have there been any fumes or chemicals near the crabitat recently?
No
12. How often do you clean the tank and how?
I usually do a big clean once a month boiling shells and such. I clean the water pools with vinegar as well. if all crabs are topside I'll clean the walls of the tank with vinegar too.
13. Are sponges used in the water dish? If so, how are they cleaned?
no sponges are used in the tank.
14. Has anything new been added to your crabitat recently?
a new shell my mom bought at a shell store at the beach but it was boiled prior to adding it to the tank
15. Is there any other information you would like to share that might be helpful (anything that is regularly part of your crab care, playtime, bathing, etc.)?
No
16. Please describe the emergency situation in detail.
about a week to a week and a half a go all the crabs in the tank started burying and staying under ground longer, I assumed they were molting as I've had 3 molt at the same time before. one or two would come up but go back down. today I went to change their water because they had kicked a bunch of sand in it. next to the water pool I noticed something weird though. it was the large claw of one of my two big crabs, S.B (soccer ball). I could tell by the claw size and leg color, she has a massive purple claw and strong red and purple colored legs.
https://imgur.com/a/yiPUfp2
I have not seen any crabs topside all day and have heard no chirping. I did see one up a couple nights ago but I was in bed so I didn't go across the room to look at him since it was late and nothing seemed wrong at that point. I am VERY scared for S.B though and the other crabs. I don't know if it's smart to dig and check on them as some may be molting, but I'm really scared that maybe some have been attacked and could be in danger, and worst case that S.B could possibly have died. Does anyone have advice on what I should do?

UPDATE: one of the crabs has come topside, turns out this one is actually S.B. which leads me to believe it is the second largest who has been injured and who's claw I found at this point. although the leg does not match that crabs colors so I am unsure if a second crab is injured.