yep a couple things you should know.
Firstly, fiddler crabs are a saltwater species, that can be
kept in brackish water, but not full freshwater. Their longevity in
freshwater is estimated at 6 months to 1 year where in salt or brackish
water can live up to 7 years or so.
Secondly, fiddler crabs need to be able to get out of the water.
You need a log, rock, beach ,etc, for them to be able to fully exit the water at various times.
Thirdly, fiddler crabs can and will catch and eat small fish, so be aware
of this possibility, as well as the fact that many fish, including catfish like pictus, raphael, synodontis, and various fish like cichlids, loaches, and some larger tetras, will hunt, kill, and eat crabs.
Hope this helps.
Myself, I would probably keep them only in a salt or brackish water
"crabitat", which consists of 50% land and 50% water.
A single pair of fiddlers would be suitable in a 5 or 10g tank by themselves.
Never to suffer would never to have been blessed.
Edgar Allan Poe