Because a molting hermit crab is very vulnerable to stress and injury from other hermit crabs (due to their inactivity during the molt and the soft, new exoskeleton), many experts advocate moving a molting crab into an isolation tank or "iso tank" during the molting process. Other people leave their crabs to molt in the main tank. A third option is to isolate a molting hermit crab within the main tank.
It is really an individual choice and the most important thing to remember is that the environment of a molting hermit crab is critical no matter which method you use.
Using Isolation Tanks
As long as you set up an isolation tank with appropriate environmental conditions (especially with the temperature and humidity) it is a good option for molting hermit crabs. The primary function of an isolation tank is to allow a hermit crab to get through their molt without the potential stress of being bothered by other crabs. You will need to obtain a second tank with all the accessories to have ready for when your hermit crab molts. Watch your crab closely for the signs of molting and if you do make sure you move your crab into the iso tank. Be prepared that sometimes you will see signs that molting is coming and move your crab but the molting will not happen for a while so you may end up isolating him or her for a long time.
Another option is to use a smaller tank (e.g. a plastic tank) inside the main tank which makes providing the same heat and humidity as the main tank easier. Place a deep layer of sand in the bottom and provide food and water in both tanks.
Not Using Isolation Tanks
The main advantage of not isolating your crab in a separate tank is that your crab will not have the additional stress of moving to a new tank during an already stressful time.
You also do not have to worry about the maintenance of multiple tanks if your crab stays in the main tank. However, this does leave your crab in a vulnerable position and sometimes your other crabs may attack a molting crab.
When crabs are left in the main tank for molting, some owners will use a cut-off two-liter pop bottle to provide a protective barrier around molting crabs. Cut the top and bottom off of the pop bottle and sink the bottle sides into the sand to surround a molting crab with a protective fence from the other crabs, especially in the sensitive post molt phase. It is a bit harder to make sure the crab has access to food and water this way but you could use small dishes (such as half shells) as a temporary measure. Be sure to remove the bottle as soon as the crab's new exoskeleton hardens and surfaces so he can get around the tank.
Even if you are watching your crab closely for signs of molting you can still be taken by surprise by a crab molting unexpectedly. If this happens, be sure to never disturb a crab which is in the actual process of molting. You can sink a cut-off pop bottle into the sand around a molting crab, making sure the other crabs can't get over or under it.
Once your crab has shed the exoskeleton and had a bit of time to recover, you may be able to scoop him up and very gently place him and his molted exoskeleton in an iso tank - but only if necessary.