How to make a brackish fidler crab aquarium setup?

A few months ago, I stupidly purchased a female fiddler crab for my freshwater aquarium. Ever since, I have been wanting to transfer her to a brackish setup, but I have a few questions. Would a beach setup work? (Half water half gravel) Would instant ocean marine salt work? If so, how much do I put in? Would a 5 gallon aquarium be big enough for 4 fiddler crabs? What temperature should the water be? What temperature should the air be? In farenheight please. Thanks!

You're right that they need brackish water to survive. Instant Ocean is good to use, and you will need a hydrometer or refractometer to measure the specific gravity. You will want to set up the new tank with fresh water and slowly bring the sanity up by about 0.002 sg per week so as not to shock the fiddlers. I think a target of 1.010 sg is good for these guys. Use rocks to support the sand or gravel and build up a land area. Also, you would need at least a 10 gallon tank with lots of land area for 1 to 3 crabs; a 20 long tank for 3 to maybe 5. Crabs are territorial and need adequate space for each to feel comfortable. Also, the heater should be set around 78 degrees and a tight-fitting glass top will keep the humidity high.