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Fiddler crabs live on ocean shores and marshes. They need half land half water setup and they need brackish water.
That website gives a lot of good information about them.
A lot of times pet stores will sell them as fully aquatic or freshwater crabs, please don't believe them. They are neither fully aquatic or freshwater.
I have two fiddlers in a 10 gallon tank, with small gravel that slopes up to form a dry area, and I keep the water about 5-6 inches high. They do spend quite a bit of time out of the water.