I currently have 3 RCC's 2 female and 1 male in a 90g planted community.
They need air or will die! They will try to climb out to get it. I have found this Atlantis Underwater Islands - An underwater pocket of air for your crabs or newts
These are great little bio-domes for the bottom of your tank attach to an air line for constant fresh air have an optional feeding tube to feed them in the habitat (crickets are fun), and come in multiple sizes you will need the smallest one for that size tank. My RCC's love it took some doing to get them to find and use it but they are almost a year old now. They will fight if you have more than one male so you know. They don't bother my plants, and will take care of any slow moving weak or fish like shrimp other than that mine don't bother anything else and they haven't even tried to climb out since.