What happens next?

How to care for pet fiddler crabs 
Provide your crabs a good home. 
Fiddler crabs should not be kept with fish. People have kept them underwater successfully, but these crabs don't live in water, they live on beaches, but enter the water as well.
Their entire tank should be damp sand, with an area of at least 1-2 " of water for part of the tank. A 10 gallon (37.9 l) tank could contain 1-2 gallons (3.8-7.6 l) of water.