You should have a cover for the aquarium since hermit crabs are talented escape artists. A sliding glass top is best because it will maintain the humidity in the cage, which is really important. A plastic cage would also be fine. If it has a plastic top it should be able to maintain humidity about as well as a glass aquarium.
Some people use wire mesh cages.
These cages give the crabs the opportunity to climb, but they don't retain humidity, so I don't recommend them. Be careful because a hermit crab would be willing to leave its shell in order to escape. The bottom of the cage should be covered in about an inch of either sand or gravel. Sand is also easier to clean. It might be a good idea to have an extra, smaller cage to use for separating crabs when one is molting.