What Do Crabs Eat in a Fish Tank? It Depends on Which Crab You Bought

You drop a pellet into the tank, and before it settles a claw snaps it up and the crab backs under a rock to eat in private. Most aquarium crabs are omnivorous scavengers. They do well on a sinking crustacean pellet as the staple, plus brine shrimp, bloodworms, blanched vegetables and algae. Feed small portions and clear away leftovers the next day. Beyond that, the right menu depends on your species. What else lives in the tank matters too, because to some crabs your fish are food.

The basic crab menu

Fiddler crabs are a good model for most pet crabs. One care guide calls them "omnivores that will eat anything they can find." It recommends a dry pellet or flake as the daily staple, with these as supplements:

• Plankton
• Brine shrimp
• Seaweed
• Blanched zucchini
• Bloodworms

Hobbyist and retailer sources suggest a specific kind of staple: sinking crustacean pellets that don't dissolve. Crabs feed on the bottom and pick at food slowly, so a pellet that falls apart quickly mostly feeds the bacteria in your tank. For variety, the same sources suggest small amounts of apple, mango, kale or cooked sweet potato. They also recommend calcium-rich foods, because a crab has to rebuild its shell after every molt.

How the menu changes by species

"Aquarium crab" covers many animals, and their diets lean in different directions:

• Red claw crabs: bloodworms, brine shrimp, uncooked fish or shrimp pieces, spinach, peas, leafy greens, algae pellets and sinking fish food.
• Thai devil crabs: pellets, brine shrimp, bloodworms, fruit and algae.
• Vampire crabs: pellets, brine shrimp, bloodworms, crickets and vegetables.
• Panther crabs: brine shrimp, pellets, dead plant matter and algae.
• Rainbow crabs: plant matter, fruit and dead matter.
• Thai micro crabs: algae, biofilm and decaying plant matter.

Thai micro crabs stand out. They graze the film that grows on surfaces and the plant debris in a mature tank, so a sterile new setup gives them very little to eat. The larger species are more willing to eat meat, and that matters for the next question.

How much and how often

Sources disagree on frequency. Some say once a day and others say twice. They agree on two rules: give small portions, and remove anything left over. The fiddler crab guide says to feed once daily and take out leftovers the next day. Don't leave food sitting for several days. Uneaten food rots and fouls the water.

A crab that is fed too little doesn't just lose weight. It goes looking for other food.

When the fish are on the menu

The idea that crabs are harmless tank mates doesn't hold up for several popular species:

• Red claw crabs are territorial and "have been known to kill other fish and animals." Their care guide recommends a species-only tank with one male to one or two females.
• Panther crabs: "If they're not fed properly, they get aggressive and resort to eating tankmates." Regular feeding is part of keeping the peace.
• Matano crabs go after snails and fish.
• Rainbow crabs can kill their own kind.

Danger also runs the other way. Small vampire crabs should be kept away from large fish that could eat them. One species guide lists goldfish, cichlids and angelfish as poor companions for crabs. Safer choices on the same list include Neocaridina shrimp, ghost shrimp, ramshorn and nerite snails, and pygmy corydoras.

Hungry crabs also wander. Pom pom, matano and gold claw crabs are known escape artists, so a tight-fitting lid is part of keeping them fed and alive.

The water matters as much as the food

A good diet can't make up for the wrong water, and some of the most common pet store crabs are sold into the wrong water.

**Many "freshwater" crabs need brackish water.** One guide says fiddler crabs, "though often sold as freshwater pets, … are not freshwater animals." They need slightly salty water. Guides commonly give a specific gravity of 1.005 to 1.010, and one says below 1.01 is best for most species. Red claw crabs are brackish too, at about 1.005. You make the water brackish by adding marine salt, not table salt. For these two species, brackish water is a requirement.

Treat tap water with a conditioner that handles chloramine. Running tap water through a filter is not the fix. Many water utilities disinfect with chloramine rather than chlorine. A product that only neutralizes chlorine can leave the ammonia half of chloramine active in your tank. Use a conditioner whose label lists both chlorine and chloramine. Boiling water or letting it stand overnight doesn't remove chloramine. Your local water utility can tell you which disinfectant it uses.

Match the temperature to the species. No single range fits every crab (all figures in °F):

• Pom pom: 68–78
• Thai devil and vampire: 72–82
• Fiddler: 75–85
• Red claw: 75–80 by one guide, 70–88 by another
• Panther: 77–86

Give land to crabs that need it. Pom pom and Thai micro crabs are fully aquatic. Thai devil, vampire, rainbow, red apple, orange arm Borneo and panther crabs need dry land to climb out onto. For red claw crabs, one guide suggests roughly three parts water to one part land. Fiddler crabs do well with a sloping sand bank and water only a few inches deep.

Size the tank to the crab. Pom pom and Thai micro crabs can live in 5 gallons. Thai devil, vampire and red claw crabs need about 10, although some keepers use 5 for red claws. Panther and matano crabs need 20. A 10-gallon tank can comfortably hold up to four fiddler crabs, with about 5 more gallons for each extra crab. With proper care, a fiddler crab can live two to three years in captivity.

One crab you shouldn't buy at all

If you shop for crabs online or at a live seafood market, watch out for mitten crabs, genus Eriocheir, which includes the Chinese mitten crab. In 1989 the U.S. Fish and Wildlife Service listed every species in the genus, and their viable eggs, as injurious under the Lacey Act. Importing, exporting, selling, acquiring or buying live mitten crabs without a permit is prohibited. Penalties include up to six months in prison and fines up to $5,000 for individuals or $10,000 for organizations. Many states add their own rules, including California, Florida, Illinois, New York, Texas and Washington.

The rules have a reason. Mitten crabs carry the Oriental lung fluke, and their burrowing can undermine levees. A single female can produce 250,000 to 1,000,000 eggs per brood. A federal crackdown called Operation Mitten Catcher stopped about 15,525 illegally imported live crabs.

Choose a legal species, learn whether it needs fresh or brackish water, and feed it small, varied meals. Your crab will likely live longer, and so will your other fish.

Sources (14)

• Fiddler Crab Care Guide – AquariumSource
• Red Claw Crab Care Guide – AquariumSource
• Red Claw Crab – Fish Tank Advisor
• Freshwater Aquarium Crabs – ModestFish
• Fiddler Crabs Care Sheet – Ward's Science (PDF)
• What Do Crabs Eat? – Aquariadise
• Hikari Crab Cuisine – Green Aqua
• Water Conditioner for Fish Tanks Guide – Gensou
• Chlorine and Chloramine – Aquarium Science
• Chloramine discussion – Hermit Crab Association
• U.S. Fish and Wildlife Service Clamps Down on Illegal Mitten Crab Trade (2021)
• U.S. Fish and Wildlife Service Cracks Down on Illegal Mitten Crab Trade (2020)
• Chinese Mitten Crab Ecological Risk Screening Summary – USFWS (PDF, Jan 2026)
• Chinese Mitten Crab – National Invasive Species Information Center