Question: When Can I Return an Isolated Molting Hermit Crab to the Main Tank?
Answer: Once a crab has molted, it takes a while for the new exoskeleton to harden so the crab shouldn't be put immediately back into the main tank. Wait until the crab is moving around well (he may even look like he is trying to escape the Iso tank) and eating and drinking normally. This could take anywere from a few days to a couple of weeks. Once the crab is moving around well and eating normally, he will be happiest being back in the main tank with his buddies rather than on his own.
You might want to give a crab that has been in isolation a quick bath in lukewarm dechorinated water (you can add a bit of Stress Coat too) just before returning it to the main tank (only once the new exoskeleton is hardened!).
Go To Hermit Crab Molting FAQ for more on molting hermit crabs