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Chances are that this is a case of wettail. Your local pet store may carry a product called Dri-Tail or similar product which is a over the counter antimicrobial product for small rodents.
However, you are correct in assuming that correcting dehydration is important. The easiest way to do that if you do not have access to a pet store, is to use a human pediatric electrolyte solution such as pedialyte. You can administer this orally (1-3 drops) every hour or so. Keep in mind, this will not likely help the loose stools, but can help support hydration. I hope this helps and give you some direction.