try the prawn in the coke bottle trick! Cut the neck off an empty coke bottle & then turn the neck around and insert it into the body of the bottle so that it is a funnel shape.
Then place a small piece of prawn in the bottle & place it on the bottom of the tank at night...
Then wait..
And wait...
And wait...
I have managed to catch a few mantis shrimps & a pistol shrimp (plus a few crabs) this way, but it takes patience... the shrimp will hapiily go in, but can't get out...
If you can't catch it first time, through the prawn away and replace it with new meat, otherwise you may have a small spike in the tank.
Regards,
Blennie...