i have a red clawed crab. he lives in a 20 gallon all by himself
he had a buddy but beat him or her up
anyway the tank is partially filled has a ton of drift wood in it and sand substrate.
they need to be in a tank where they have access to the land and have hiding places above and below water. they also prefer sand and actually are really good for sand as they turn it. mine eat crab pellets, bloodworms, pond snails,brine shrimp, shrimp pellets, and he loves raw peas. he also eats any plant i put in there for him
they also prefer brackish water as the salt helps harden their shells when molting.
you need to also make sure you change the water atleast wekly as they are pretty dirty little buggers and create alot of waste. the most dangerous time in a crabs life is when it has molted or is molting, they are very suceptible to disease and stress at this time and need to be given extra care during this time making sure they have places to hide and get above the water if need be and making sure their environment is clean.
ideally to have a healthy crab you want to have them in an environment that resembles their natural environment of south east asian mangrove swamps. this will help lower stress and help keep them happy and healthy.