So my wife and I purchased a nice aquarium, couple of nice fish and this crab. We look up how to care for the crab on the internet (granted, we should have done this first) and, lo and behold, carbs need all kinds of special care (A spot to sun themselves, saltwater etc). So, I have two questions that I was unable to find on the internet. One: can I expect this crab to live if he has no saltwater and no place to get out of the water? Two: does the water he uses have to be heated? I ask this because I have the idea of making a little “crab condo” which I will hook onto the side of the exsisting tank which the crab will be able to climb into via stratigically placed plant (we know he can do this because he has already climbed out of the tank using plants and the heater) If I make this external tank, it will have no heater and no water circulation, but it will only be for him to climb out of the water when he needs to, he’ll still have access to the big tank and all the food and heat he wants. Does this sound like something a crab could live with?
This site should answer your questions, it does appear that they do need to live in salty water or “brackish water” (not as salty as sea water)and can only survive in freshwater temporarily.
Many pet stores will sell animals like fiddler crabs which survive in aquariums just long enough for the buyer not to get too mad at it dying too quickly. This is why you have to research animals before buying them, to find out if it’s even something that can live in your tank.