Fiddler Crabs Need Brackish Water, and Why Most "Freshwater Crab" Setups Fail Them

Fiddler crabs are fun to watch. The males wave one giant claw like they're conducting an orchestra, they dig burrows, and they scurry around in groups. They're also one of the most commonly mis-sold animals in the aquarium hobby. Pet stores often keep them in fresh water and sell them as freshwater crabs. In fresh water, they slowly die.

Where they really live

Fiddler crabs live on mudflats and beaches where rivers meet the sea. The water there is brackish: part fresh, part salt. At low tide they're out on the sand feeding, and at high tide they retreat into their burrows. They spend most of their lives on land, not underwater.

What goes wrong in a "freshwater" tank

• No salt. Fiddlers need some salt to keep their body chemistry balanced and to molt properly. In pure fresh water they can survive for a while, then decline.
• No land. They're semi-terrestrial. A fully aquatic tank, even one with a rock to climb on, can drown them.
• Bad molts. Crabs grow by shedding their shell, and they need land, salt and somewhere to hide to do it safely.

What a fiddler crab tank needs

• Brackish water. Use marine salt (the kind for saltwater tanks), not aquarium salt or table salt. Most keepers aim for a specific gravity of about 1.005 to 1.015, measured with a hydrometer or refractometer.
• Lots of land. Many keepers do half land and half water. Others go mostly land with a shallow beach of water a few inches deep. Either way, make the sand deep enough for burrows, several inches above the waterline.
• A gentle slope from the water up to the land, so they can walk in and out.
• A tight lid. They're escape artists.
• Warmth: mid-70s to low 80s °F.
• A group, with more females than males. Males fight over territory.

What they eat

They sift the sand for bits of food. Feed sinking pellets, crab food, algae wafers, and an occasional piece of blanched vegetable or frozen shrimp. Our guide to what crabs eat in a fish tank has more.

Can they live in my river tank?

Not in a freshwater river tank. They need salt, and most river fish don't want it. A brackish riparium is possible, with a beach and fish that handle brackish water, but it's a separate project.

Crabs that really are freshwater

If you want crabs in a freshwater riparium, look at vampire crabs (Geosesarma). They're small, colorful, semi-terrestrial, and live their whole lives in fresh water. They still need lots of land and a humid lid. Red claw crabs, another common pet-store crab, are also brackish and have the same problem as fiddlers.

Learn more about how crabs handle water and what crabs eat.

A lesson from our first tank

Crabs were part of our original river tank, and they taught us that a crab will escape through any gap in the lid. Read the story on our About page.

Sources (3)

Aquarium Breeder: Fiddler Crab Care Guide · PetGuide: Tips for Keeping Fiddler Crabs in Your Brackish Tank · Wikipedia: Vampire crab

Photo: Fiddler crab 2 by Wilfredor, CC0.

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