Don't buy the walmart crabs! Those are actually fiddler crabs! I bought one, and put him in my freshwater community aquarium and fell in love with him! He was so great! I even followed the guys directions and took him out for about 4 hours every week to let his shell harden! Yea, well because he was a fiddler crab, but walmart didn't call them that, and I had no area for my crab to crawl out of the water (because he was sold to me as being able to live in water) he died two weeks later. Fiddler crabs need to be able to crawl out of the water whenever they want, so species tanks are easier to do with them. I did a lot of research after little altoid died :( This was last year, and I impulse bought so it was my own fault. *Sigh. Anyway, I thought you should know that the walmart cuties are fiddler crabs.
"Just keep swimming, Just keep swimming, just keep swimming swimming swimming! What do we do? We swim" :)