Red-clawed crabs (Perisesarma bidens, formerly Sesarma bidens) are often sold as "freshwater" crabs, but really do their best in brackish water. They are widespread in Asia, where they dwell in costal mangrove swamps. They can survive in full saltwater and freshwater, but do their best in a brackish aquarium.
Red-clawed crabs do their best in a tank arranged just for them. A 10-gallon aquarium, half full, makes a great crab vivarium. Sandy substrate works better than gravel. The crabs should have perches above the waterline. Aquarium driftwood sticking out of the water makes a great perch. Additionally, crabs may get supplemental nutrition from eating bits of the wood. Last but not least, a tight-fitting lid is critical to keep these crabs from escaping and winding up desiccated.
These crabs come from a very wide geographic range. As such, they do well in a variety of conditions. They have a tropical distribution, and do their best at temperatures of 68 to 75 degrees Fahrenheit. Red-clawed crabs prefer a pH of 7.4 to 8.2. Despite some debate on the matter, these crabs do their best in brackish water. This means water with 1 tablespoon of salt per gallon. This should be aquarium salt or kosher salt, not table salt.
Part of the red-clawed crab's adaptation to such a huge area is their ability to eat a wide variety of foods. Red-clawed crabs will eat just about anything. This includes shredding aquarium plants and slow-moving fish. Shrimp pellets and other sinking prepared foods make a great staple food. These foods are available at most pet shops.
Red-clawed crabs require some considerations where tankmates are concerned. Avoid multiple male red-clawed crabs in the same aquarium. Males have larger claws than the females of the species. Fast-swimming fish that live in the upper levels of the water are decent tankmates. Bottom-dwelling fish, particularly slower-moving ones, run the risk of winding up as crab food. At the same time, fish large enough to eat these crabs should be avoided.