I keep the Red Claw Crab, Sesarma Bidens or Gaectices Depressus depending on who you ask.
I have found they are excellent pets, once you can accomodate their needs. Most crabs are land based, the Red Claw can live under water indefinitely but will fare much better when they have access to both land and water. They likewise cannot live on land completly as they need to be submerged for the new shell to harden after a moult.
This is one of the interesting features of crabs, they wont grow in size in front of your eyes, instead they shed the old carapace when the new shell is almost fully formed underneath. This then hardens over time and within a day, they emerge larger and hungrier than before! The old shell looks exactly like a crab, a few shades lighter in colour...
They can live with fish, mine shared a tank with most of the fish listed below and more and I have never had casualties. Male crabs will fight amongst themselves, the best arrangement is one male and two female crabs. They become very shy and hide most of the day if they are kept on their own. They communicate by waving claws and banging the ground, its great to watch!
I have found they LOVE Hikari wafers for Cory's, bloodworms, brine shrimp and some veggies. Also some beech tree leaves that have withered and completely brown help them to fight infections and keep themselves healthy.
They require medium to hard water, a PH above 7 and some prefer but dont necessarily need salt in the water. The Red Claw is born in almost sea water but migrates inland as they grow older. If they are kept in soft acidic water they wont be able to grow a healthy shell and may lose limbs.
Like a lot of invertebrates, they are very sensitive to nitrAtes, more-so than fish so be careful here.
If you are picking a crab, look for one with clear eyes, ther should be a dark spot surrounded by lighter coloured tissue like a human eye.
If they are missing a leg or two, they will grow another at next moult which happens every 6-8 weeks in young crabs to 3-4 months for older ones.
Males will have larger claws but the sure way to sex them is by looking at the flaps underneath, males will be almost pointy at the front, where a females is fully rounded, almost reaching where the legs meet the body.
A good 'land' area for the average fish tank is a piece of cork with a plastic plant or some sterilised rope to climb aboard! They are very good at escaping but after spending some time in the tank are more inclined to stay put, mine stopped trying to escape after 6-8 months.
Like I said earlier, they are great pets and get up to some funny antics, my female Suzi re-arranged my carefuly laid out planted tank to a set-up more to her liking while we were having dinner the other night with freinds, they were still talking about it in the pub later on that night!
Good luck, if you need any more info, just ask!
Ken
