Since there are about 12 million fiddler crabs dieing every day because of uneducated owners,
i decided to make this instructable
Most of the problem is about how pet store employees are miseducating buyers and telling them the exact opposite of supplies...

Step 1: Supplies

Here are the basic needs for a fiddler crab
-A 5 gallon tank (the bigger the better)
-a heater (Yes fiddlers get cold)
-A filter (Go for the internal ones for fiddlers because fiddlers are great climbers and can crawl up the intake tube and escape)Never go for undergravel filters for crabs...
-Sand (Fiddler crab legs are designed to crawl on sand,don't go for gravel because it makes it hard for them to crawl on and believe it or not fiddler crabs will die of frustration)
-Sea Salt (Fiddler crabs live in brackish water-half salt half fresh-and will die in months if in freshwater)

Note:Fiddler crabs are NOT semi-aquatic,remember they live in beaches not the ocean

Step 2: Determining the Gender

The gender of a fiddler crab is easy to determine,male fiddler crabs have on normal claw and one big claw,while females only have two normal claws

see picture below

Step 3: Choosing a Crab

Choose healthy, active fiddler crabs with all their limbs still intact. A good set-up is one male to about three females. Crabs are sold in many pet stores. People also like to catch fiddler crabs on beaches.

Step 4: Feeding

Feed your fiddlers algae disks or commercial crab food. They also find shrimp pellets and frozen bloodworms quite tasty. A slice of zucchini is an occasional treat. Remove uneaten food after 24 hours, or you'll be growing mold and smells.

Step 5: Heating

Keep crabs comfortable in a room-temperature (75-85 degrees F) environment. If the water is too cold, invest in an aquarium heater or basking bulb.

Step 6: MOLTING

When fiddler crabs molt they will eat very little and will stay upside down for many hours, DO NOT touch or mess with your fiddler crab when its molting! Fiddler crabs may look hard but are acutally VERY FRAGILE!
I once accidentally KILLED one of my females when I tried to help it back up...
After your fiddler crab is done molting you may see him eating the old shell THATS OK,fiddler crabs will eat the old shell for nutrition,DONT TAKE OUT THE SHELL EVEN IF THERE ARE NO CRABS AROUND IT! Your crab needs the nutrition and will only it the shell when no ones around (exception for other crabs)
Be sure to isolate your fiddler crab when hes done molting,after he starts to be active and his new shell comes in you can put him back in the main tank...