Hermit crabs are pretty vicious to each other, especially if one is at a disadvantage and can't fight back as well. QT
or sump sounds like a good idea, but you need to make sure that QT
is cycled first, or be prepared to make regular water changes. You also need to be aware that if you have used a copper treatment in that QT
before, it could be bad for your hermit.
I would use aquarium water in the QT
, and put the new water back into the aquarium. That way you get a small head start on the cycling process. To get the cycle started, add a small amount of food or some other ammonia source. A proper cycle can take weeks, I don't know as if a couple days is going to be useful.
If you have to put him in the QT
right away, test the water for ammonia daily, and change the water as necessary to keep the ammonia levels down. Use aquarium water for your water changes. (If the QT
does not have any copper or other contaminants in it, you can put the QT
water back into the main tank, where the bacteria there will process the ammonia. You can save a little on salt that way since this is not a case of diseased fish.) Be extra vigilant about removing uneaten food and other wastes.