local beach crabs in a reef tank

[gregzz4]

I'll say it again ...
Adding local shore crabs to one's tank is a risky business.
You run the risk of adding pathogens/diseases whatever to your tank.
And now are potentially adding them to the reefing community.
Diseases, pests and so forth come from somewhere;
Flatworms, nudibranchs, whatever.
We all do our best to keep them out of our tanks.

It would be different if these critters were coming from the same places we buy our crabs and shrimps from

You have NO idea what you are adding to your tank by adding crabs from your local shores.
And by doing so, you are adding them to our community.
Have fun with your crabs and I hope you don't incur any issues ...