If you stare at the tank long enough at night, you will eventually locate the spots where the mantis are lodging.
Removing the rock will help (if you haven't bolted them into pvc or anything).
The next steps were what I did (I caught 3 mantis this way) - again these are the smashers, not the pinchers. Just in case anyone reads this thread and has no idea where TBS is from (gulf of mexico rock has smashers...its the pacific with the spearers).
Have a bucket of saltwater to place the rock in. Hopefully it's small enough to fit.
Have a cup of saltwater ready, and when you lift the rock out of the bucket, pour the saltwater into the hole. This exercise is to see where the water exits. It's possible there is a backexit that the mantis will come out of. This is so that you don't get startled by the green mantis when you use (approx 5 ounces or so of) fresh water. Anyways - if you determine the hole is a deadend, then clearly it's going to come right out at you.
Next pour the freshwater in - be prepared - they are very fast.
All of mine came out and plopped into the bucket of saltwater. The fresh water should not be so much that it will impact the salinity of the bucket too much. This way I was able to save them for the brooklyn aquarium society. You could possible save them and turn them to a petshop or club nearby as well.
Feel free to feed them your whelks and xanthid crabs. It's amusing to watch them ambush them. Be sure to remove the crabs once eaten though as they can foul whatever your keeping the mantis in. To be safe you may just want to feed them a bit of shrimp.
By the way...less than 1 inch pistol shrimp are extremely loud in a glass cage. Pat had some huge one(s) in her tank. I only had these tiny clear ones. Too small to impact any of the cleaning crew. Pat's on the other hand probably needed to eat something...
Finally - most of the mantis from the TBS rock is not big enough to "thumbsplit" your fingers. I never tested it - but they are pretty small compared to say the peacock mantis.
mine were all about 3~4 inches in size...(sorry old camera, old pics)
