HCA: Hermit Crab Association

so I have a usually a very active hermit crab but nowadays... he just sits there in one spot. He moves to another spot in a couple hour and jsut sits there and rinse and repeat. Umm.. is something wrong with my crab? My humidity is usually 70%. Whatever I do, I can't raise it above 70%... is it my humidifier? I use an almost completely sealed top too so iunno.. the temperature should be fine.... I feed it a mix of foods with shrimp, pellets (from a trusted internet shop) and etc. I have salt and fresh water pools. Substrate is like... 4-5 inches and he often digs around in that stuff... well he did 2 weeks ago. But now meh.. he just stays in one place at a time. Did he find like a favorite spot or something or is it something I should be worried about. Well... he stays outside often now... he doesn't really hide in his cave

Does he have a friend or two? He could very well just be lonely, hermit crabs are very social animals and prefer the company of other crabs, contrary to what their name would lead you to believe. The humidity is a little bit low... I would try putting in a moss pit, that did wonders for my humidity levels. A bubbler in the fresh water pool will help too, as well as helping to bring fresh air into the tank at the same time. Also, what exactly is the temperature in there? I find that temperatures near and over 80 encourage them to be more active.

ahh I see. I boght a friend for him at the store but he introduced mites to the area. I redid my tank cuz of that but I guess I should... so how should I go with finding a new one? Got adive for me? I have a 10 galon atm

So do you still have the other crab you bought, or did he not survive?
To find more crabs, the best thing would be to adopt, or find unwanted crabs that you could get. Lots of people will lose interest in their crabs after getting them as an impulse buy or just not have enough time to continue caring for them, and will post adds on craigslist and petfinder - look on there in your area, as well as in the adoptions topic on this site. Find the post with your state and see if there is anyone with a crab or two to give up. Keeping in mind that you only have a ten gallon tank, I wouldn't get more than 2 or 3 smaller sized crabs. Make sure you consider how long they will be in that tank, keeping in mind that they never really stop growing.
If you don't find any crabs that way, try to find a different local pet store than the one that the crab with mites came from. Small family run pet stores usually are a better bet for getting healthy crabs than big chain stores like Petco and Petsmart.

It died a couple days after I bought it. I don't know why... I really dont... ;3;
He started losing legs and stuff... it was scary.

Also.. I'm actually starting college in a couple months so it'll be hard taking care of these guys. I'll be trying my hardest though. But yeah....

I would be worried about airflow, CO2 tends to sit at the bottom where he is, and if the top is completely sealed, how is fresh air coming in and stale air getting out? That combined with yes lightly low humidity might be making him a little sickly :/
As a side note, they do often have "spots" but the lack of activity worries me.

What is the temp in the tank?

Lots of crabs don't make it through the first few days after you buy them, too much stress is lethal for them.

I'll be starting college at the end of August too, and I've been teaching my parents how to care for all my critters haha.

Have you recalibrated your hygrometer? That could be the issue and, if it is, you could be overdoing the humidity.