from Yote "The next mantis you get,place your net in the tank with a silver side in it.Do that daily until the mantis get comfortable eating out of the net.Then tie a string or fishing line to the handle of the net once the shrimp gets comfortable,then its as simple as sitting back and setting the hook."
Make sure you get him on the first try though, because he won't fall for it a second time.
If you do a FW dip make sure you use RO water, if not it could cause an algae outbreak. Also, if you know exactly which hole he's hiding in, you could boil the RO water and use a baster to shoot it right into his hole, he should either get boiled or come right out.
As far as the depth of a toadstool, it really depends on your lighting. They prefer moderate/high lighting so it would be a different heights if you were using MH as opposed to PCs