That's a slightly controversial topic, Accordz...
By far the bulk of the information on fiddler crabs says they need brackish water, and that they need a paludarium (half water, half land) with a sandy bottom to thrive. However, there have been many reports of people keeping fiddler crabs in brackish or midly brackish aquariums with only a rock or a hunk of driftwood sticking out.
Check out my setup for my crab here:
http://www.myfishtank.net/forum/showthread.php?s=&threadid=22947
So far, all is good... thanks to that rude advice by that "expert" spanish guy--now banned--I added another tablespoon of aquarium salt for good measure. (By the way, I'd take everything that guy has to say with a grain of salt... I did a little searching on this forum and read some of his older posts under another name, which was also banned, and they were... let's say, interesting.)
As for redclaw crabs, largely, they are considered to be total aquatics, but there is some controversy there also. They also climb up for air and will escape your tank if they are given an escape route. As for water conditions, some say brackish, some say fresh... it's not really concrete.
Here's a site for more info.
http://wrongcrowd.com/aquaria/crab/
And Jonny Sqee, a crawfish may not be the best bet for your tank if you're planning on keeping smaller fish. I think most species of crawfish are avid fish eaters.
I'm not the best person to dispense out advice on aquatic or amphibious crabs... hopefully, someone who's kept them for years--and know what they're doing--will step up and shell out some info.