Hi, new to hermit crabs here. We got 2 for my daughters about 3 weeks ago. After doing some research, I realized we needed to upgrade their habitat, so bought a used set up off Facebook Marketplace - 10 gallon glass covered tank with lots of nice sand, hiding structures, water/food bowls, heater and thermometer and humidity guage. I get home and put our crabs in and they seem to love it. They start digging and my daughter finds some buried shells, and it turns out the prvious owners hermit crab is in it, alive and well and comes out and walks around the tank!!!
I reached out to the person I bought the tank from and they said they were shocked and hadn’t seen the hermit crab in months, and their may be another one in there also

I suggested to them that they may have been molting. They seemed embarrassed and said to enjoy our crabs new friends.
SO, I kept watch and all 3 crabs seemed to get along fine last night. This morning that new crab and one of my old ones must have dug under the sand.
Any tips or what should I watch out for given I now have 3, possibly a 4th), hermit crab?

Is a 10 gallon tank too small now? Should I consider adopting out theses new crabs?