New crab owner, need some help

[Terrae]

Hello! First time crab caretaker, it's been ~2 weeks and I'm having some issues.

For reference: 2 crabs, seem to be PPs, a bit over golf ball sized. 40 gallon tank, glass lid. Heat is at steady 80F from 2 UTHs attached to the sides above substrate, with thermostat. I've never seen humidity meter drop below 90% so mold is starting to be an issue but they should be breathing fine. I don't have a night vision camera yet, but can track their movements roughly from checking in the morning what has been moved around.

The substrate is the big problem. It seemed perfect at first, but it's compacted more than I expected so now it's too low. And despite the humidity and frequent spraying, it keeps getting drier, particularly on the warmer side of the tank. It currently still technically passes the finger test, but any hole wider than that is prone to collapsing, and it seems to be getting drier. I don't think I can currently replace or add substrate, or add water in any way that risks flooding, since one of the crabs has disappeared underground for over a week, so is quite possibly molting. What should I do?

Less serious an issue, crab #2 is incredibly skittish, only emerging in pitch darkness and immediately seeking cover if there is ever enough light for the slightest human vision. I knew they wouldn't exactly be snuggly pets, but it feels bad to be apparently so scary to them. Anything to be done here?

Finally, should I get another crab? On the one hand, I feel like I shouldn't get another when I'm already having trouble, and according to some numbers the tank is a bit small for three. On the other hand, with #1 (presumably) molting, #2 currently has no others for socializing, not to mention it's up in the air whether #1 will survive at all.

[curlysister]

Welcome to the forum and to crabbing!
You don't need to get another crab - if you keep adding whenever anyone is underground, you will quickly have a lot of crabs! LOL! Crabs aren't actually as social as once thought - the behavior is more about resource sharing than socializing. So a lone crab is totally fine, especially if it is temporary with the other being underground.
What did you use for substrate? Play sand or coco fiber or a mix? To check how moist the sub is, put something long and thin like a chop stick down along the glass to the bottom - if the hole holds it's shape it is just fine. Often the top inch or so will be dry, but the lower portion will hold shape just fine. Of course if there is water pooling in the bottom of the hole, your sub is too wet.
Some crabs are just more skittish than others. Be patient and ensure they have enough hiding spots, and they might get more used to you and not be as scared.

[Terrae]

Thank you for the reply!
I wasn't planning on getting a new one every time on le was underground, lol. The thought was just to make sure there were at least two active at any given time, for socializing. But it sounds like that isn't of much concern.
I did my best to make the recommended 5:1 play sand to coco fiber mix. It's entirely possible I messed up the measuring or got poor quality fiber or something, though. As for whether a long thin hole holds its shape, the answer is "just barely". If there is the slightest disturbance, it's prone to collapse. Certainly not something I'd consider good sandcastle material. There are currently zero signs of flooding or pooling, all my spraying seems to cancel out with the top layer drying.
There are a variety of hidey holes, but probably more foliage and ground cover would be good. I'm planning on finding oak leaves and branches this weekend.

[curlysister]

Hmmm, where is your thermometer/ hygrometer with respect to the UTH? What are you using for a lid?

The ratio isn't important, sub can be all play sand, all coco fiber, or a combo of the two in any ratio - so you won't have messed it up no matter what you did.