I never had any interest in hermit crabs, but my neighbor basically had a crab that needed rescuing, so here we are. I am particularly worried about this crab (Carl). I’ve been reading through all the information here, and it has been EXTREMELY helpful, but also overwhelming 
I would feel much better if I could ask some questions directly. I’ll share the setup etc first. Carl was living alone in his 10 gallon tank for about 3 weeks after his crab roommate died. No heat source, and major lack of humidity with only a screen top. Substrate was about an inch or so of dirt/mulch. He had ONLY saltwater, which they were making with pink Himalayan salt. There was a hide, and a couple shells and air plants. Food was the conventional hermit crab food, and I did see a dried up apple slice in there, though the kids said they never saw him eat or move.
So we bought the basic supplies, along with 3 other hermit crabs (Jerry, Enchilada, and Timmy), and brought him home. I have a 20 gallon high tank (I read they like to climb so I prioritized height over length), substrate is about 5.5 inches of sand/eco earth mixture, there’s a half coconut shell hide, saltwater, fresh water (I use distilled for both… will change this to dechlorinated), a couple fake plants, and several shells that I just added 2 nights ago. There’s also a coconut fiber sheet on the back wall for climbing.
I use a ceramic heat emitter, and temperature is set to 81°F, though I have an Ultratherm UTH on the way. I use a UVB light during the day, and a black light for a few hours at night to observe any activity without the need for a bright light. I turn this off before going to bed.
For the first couple days I didn’t know they needed high humidity, and I just had a screen top. Since then, I’ve put wet towels covered in plastic wrap and foil on top of the screen, and am spraying a couple times a day. This has kept the humidity close to 80%. I am in the process of getting a solid top.
The pet store crabs were pretty active the first night, but 2 of them burrowed sometime in the night and I’ve only gotten a glimpse of each of them one time since. The other stayed on the surface for a few days, mostly hiding behind the plant, but then he also burrowed 2 nights ago.
Carl has been on the surface the entire time. The first 1.5 days, one of the other crabs was very interested in him and kept grabbing him and inspecting him, etc. But aside from his first night here, I’ve barely seen him move. He went to the corner a few days ago and has stayed there. We picked him up 2 nights ago to make sure he was still alive. I put him by the food and saw him nibbling on a walnut a little later. But after a bit he went back to his corner and still hasn’t moved. I thought about putting him in the fresh water, but wasn’t sure if I should. I feel so bad for him. Anything I can do for him? We haven’t handled him at all aside from checking on him the other night.
Food and water dishes: I feel I need a better setup. The dishes are taking up so much of the floor space, and I think the water dishes should be deeper, but I worry about the little guys being able to climb out. I searched for pictures in here as examples but couldn’t find what I was looking for. Can I get some guidance on this?
Cleaning decor etc: I have a hammock, a couple more fake plants and a hanging coconut hide I want to add, but am not sure the correct way to clean them first. I also have a piece of choya wood coming tomorrow that I’ll want to clean as well. What’s the best method for these?
New shells: just boil for 5 minutes?
Lid/cover: as I said I now know I need a solid cover. I wanted glass, but I’ve had a terrible time finding exactly what Im looking for. I know there are debates on UVB, but I want to use a UVB bulb, so it has to be something that will accommodate a light UNDER the lid. I’ve seen some people use corrugated plastic board. Is that a good option? That’d be great because it’d be easy to cut a hole to fit the lamp then just tape it in place and to seal any gaps.
Hygrometer: I read that Acurite is recommended. I searched on Amazon and there are SO. MANY. OPTIONS lol. Any one in particular?
Night temperature: I’ve been lowering the tank thermostat at night. Should I do this? Should I leave it the same? Should I turn the heater off altogether?
I’m sorry this is so long! But I wanted to give a thorough description. I appreciate and am grateful for any help
Let me know if you have any questions!Sent from my iPhone using Tapatalk
I don’t know how to help him 
They do still produce a bit of heat, though. Should I be concerned about melting?
I can’t see him anywhere so he must’ve burrowed. This makes me feel a bit relieved that he’s engaged in natural behavior… but also a bit worried that he’s just gone down there to die. He looked a bit weak and unsteady when I saw him walking last night
