As a matter of fact, there's no standard amount of calcium you should give to a fiddler crab. It's advisable to ensure there's some amount of calcium in every meal. You can also place an egg shell in the water. Allow your fiddler crab to eat the exoskeletons it sheds during molting for additional calcium. Check out http://www.ncf.edu/msoi-fiddler-crabs for more details.