Setting up a proper habitat for fiddler crabs can seem daunting at first. There is very little information on the proper care of fiddlers. Coupled with the vast amount of misinformation on the web, care can be a challenging task.
First of all, fiddler crabs need a place to live in. An aquarium, terrarium or other container can be suitable. As fiddler crabs are benthic (live on ground), surface area is more important than volume. I would suggest no smaller than a 10 gallon aquarium for a fiddler crab. The larger the enclosure, the more comfortable your fiddler crabs will be, and the more crabs you can add. A 10 gallon tank will house up to 4 fiddler crabs in ideal conditions. Factors on the number of crabs your tank can house include surface area, hiding spaces, etc. Each crab should have a hiding spot for its own, and roughly a ½ square foot of land space.
In the wild, fiddler crabs come from marshes, swamps, shores, lagoons, mud flats, etc. They are semi-terrestrial creatures, and need access to BOTH land and water. Without either, they will die. There is debate to the proportion of land to water in captivity. 50% land and 50% water is a good starting point. You can create a land section by sloping substrate out of the water, a plexiglass divider, or a second level. Using plexiglass to divide the tank will allow for a larger water volume, thus you can increase the percentage of land. If using a divider, make sure both sides are easily accessible. They prefer not to climb, so a sand slope or ramp should still be provided.
Fiddler crabs are found mostly in sand or mud. In captivity, sand is best. Many fiddler crab species will burrow, and need sand to do so. Gravel is not suitable as the majority of the substrate as it does not allow burrowing, nor sifting. Fiddler crabs are detrivores, and many will sift through sand to find food. Aquarium sand, play sand, pool filter sand, and other types can be used. Remember to use sand free of chemicals such as pesticides, anti mold agents and copper. In large enclosures,
diverse substrates such as coconut fiber and natural gravel may be supplied to simulate their natural environments, but keep the majority of the substrate sand. Gravel may be used in the water section for easy of cleaning.