Fiddler crab habitats vary in salinity. Fiddler crabs are found in conditions from freshwater to fully marine. Often, most fiddler crabs sold in stores will survive in neither freshwater, nor marine conditions in the long term. Unless you know your specific species, and it’s natural conditions, a specific gravity of 1.005 to 1.015 is best. 1.010 is a good starting point, as species from either end of the spectrum will adapt. Remember, these brackish conditions are a must for fiddler crabs. No fiddler crab will live to it’s life expectancy in freshwater.
Brackish water can be made by adding marine salt to tap water. Tap water or reverse osmosis(RO) water may be used, but remember to dechlorinate tap water! Remember to use marine salt mixes for saltwater fish, not sea salt or table salt. You need to calculate the required amount of salt for the gallons of water you are putting in. Replenish the salt every water change, but do not replace evaporated water with salt water, as this will increase the salinity(salt minerals do not evaporate). To calculate the amount of salt required, look at the instructions on the salt mix. If it says to add ½ cup of salt per gallon for a salinity of 1.022, then you will need about ¼ cups of salt for 1.010 salinity. It may be easier to convert the units to teaspoons or tablespoons. Brush up on your algebra. Add the salt to the water and mix well, before adding to the tank.Aerating and mixing with a powerhead and air-pump for 24hrs is best. Salt is caustic and undissolved salt will burn your crabs.
The water section will need to be filtered just like any fish tank. An internal filter works well, as power filters may not be able to function in elevation from the water. A canister filter would work as well. You will need to cycle your tank before purchasing your crabs. Crabs, like most organisms, produce ammonia in their waste. Ammonia is toxic to aquatic life, and your crabs will eventually die from exposure. Luckily, there are a group of handy bacteria which convert this harmful ammonia. It is converted first to nitrites, which are also harmful, and then to nitrates, which are only harmful in large amounts. This process is known as “the cycle”. You can find an excellent article on how to cycle your tank here.