However, it's important to consider that bigger is always better. A larger tank provides more space for the crab to explore, burrow, and climb, and allows for a more stable environment.
If you plan to keep multiple fiddler crabs, the tank size should be scaled up accordingly. A good rule of thumb is to allow 2-3 gallons per crab.
Ultimately, the best tank size for your fiddler crab will depend on your specific setup, the type of crab you have, and your budget.