Hi and welcome!
I just joined yesterday, but I've been hanging around for a while now. I recognized your setup as the one I originally had, and I want to try to help.
As soon as you can, I highly recommend a bigger, better tank. That one wasn't big enough for the one crab we had at first, and all those air holes make it really hard to keep the warmth and humidity in. First thing I would do to help this is to put a warm, wet towel over the top to help with the humidity. It's not a long-term fix, but it'll help in the mean time. Make sure to keep it moist or it doesn't do a lot of good.
Even though this will help with your humidity, I still say to spritz them often with dechlorinated water. They have gills, so they need that moisture all the time to breathe. The gauges are important, but you probably won't have the space to put them in that little area. So if you do get them before you get the new tank, you can put them in but don't make it permanent unless you want to buy another soon.
I agree about getting a heat source, but only if their space isn't warm enough. I think I read that they like a temp of 75 at least. It's heading into summer most everywhere, so if it's warm enough you may not need one yet. You will by winter, but the big glass tank would be higher on my priority list right now.
I'm sure your tree falls over if they try to climb it, ours did. I took one of those 3M sticky-things that don't wreck the wallpaper and I stuck it on the clean tank floor. Then I stuck the tree to it and covered it with a whole lot more sand. Mostly I didn't want to stick the crabs to it, but wanted the tree to stick to the "floor". Works pretty well. The crabs can climb it without it falling now.
But the only reason I had a crab in that small space was because we had an emergency surface molt and that was the only other crab appropriate place for the one who wasn't molting. As hard as I tried with that setup, I really found it just doesn't work well. It seems great at first, but the more I read on this site, the more I realized it wasn't going to work long-term.
I hope that some of this helps, and hope that ou have a lot of fun crabbing. Also, have you named your little guys yet?
Again, welcome, and have a great day!
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Gee, I guess you have a lot more info than I started with! Next time I'll try to type faster!
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They are very cute! And it seems they will have a great place to live and play with all that space. Sand is awesome, but avoid Calci-sand- it clumps and molds and is just not easy. Play sand really is the easiest- I just switched mine over and it's so much better! Have a great one!