06-21-2013, 11:32 PM
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Welcome to the forum
As someone who has kept crayfish with fish for several years, I am certain that the crayfish will eventually eat all the fish in your tank. One must really consider the nature, behavior and attitude when choosing fish to live with a crayfish, and unfortunately the fish you have are just extremely easy targets, especially in a 20 gallon tank.
I would absolutely return the crayfish.
I can't say anything about the crabs - never kept them, don't intend to ever keep them, don't know a thing about them.