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Aren't those better off in saltwater? Besides, I'm not sure if they'll get along with our feeder fish and aquatic snails. (We have some ten cent feeder fish that are like six inches long now)
Most pet hermit crabs are terrestrial. Well, for the most part. You need to provide lots of water - salt and fresh - and they like to fill up their shells, but they're air-breathers and so can't (I don't think?) live underwater full-time.