Do they smell bad? I've heard that since they need a place to come out of the water, they have a terrible smell.

Crabs in general smell terrible if their waste is allowed to build up. Every time I open a bag of them when they come into my store, I hold my breath since invariably it's going to smell disgusting. The problem with crabs however is that they really do require a specific setup, and unless you've got a decent sized aquarium, it's hard to keep both and and fish in the same tank. Add to that the fact that they'll eat fish, and it's just plain easier to keep them by themselves so you can observe their behavior.

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Wow, that sucks to hear. I know about their specific setup, so I was thinking of keeping them in a species only tank.