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You need to trap him. Could build a trap out of a plastic pop bottle.
http://www.youtube.com/watch?v=LuA97UniMd8

Lol, don't freak out too much. I had living in my tank for over 2 years....I never did figure out why he suddenly disappeared.

They will eat all your hermit crabs and shrimp so you might want to remove them while you try to trap it

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how big was it?

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sorry about your crab ...as I suspect it is done for, but hey I would love to be in your shoes. a free mantis sounds like a great deal.

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we will take him

Oh well, not fast enough I would have taken it too but good luck catching it!

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I had one in my tank until about 2 weeks ago. He caused absolute carnage in my tank and would stack the shells of his kills right at the front of my tank:cry: He also took out a few gobies and peppermint shrimp during his reign of terror. I set up a few traps with no luck and the only way I caught him is I found the rock he made his home in and pulled it out when I saw him in it and now hes living in my sump.