Its possible its already cycled, esp. if the rock was cured. Its good that you're going to wait to add fish, as the tank matures a bit more.
I would add some snails first, before crabs. Many reef aquarists no longer add crabs to their tanks, because of their predatory nature. I still use scarlet hermits in mine and have a couple old left over blue legs, but would not add them again. {blues}.
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Doug
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