With 240lbs, you will get some critters. I would not FW
dip the rock, you will loose too much good live. You can use the SW
soda water dip if you must dip the rock. You could also add it to the tank and catch the critters later.
If you choose to do the soda water dip, here is how it is done.
Get unsweetened soda water at the grocery store. Make sure you get enough to fill up a 5 gallon bucket. Fill it with the soda water and add SW
mix to the normal SPG.
Have another bucket standing by with normal salt mix for your rinse and a container for the desirable creature that may come out of the rock.
Place each piece of rock in the SW
soda mix for 1 minute. The critters will come jumping out. You then rinse the rock in the normal salt mix and quickly place it in the tank. Quickly remove the wanter critter to the container for later placement in your system.
Keep doing this until the soda water looses its carbonation. You may then need to mix up another bucket. The salt mix seem to lessen the live of the soda water. It's a pretty good method, but I still missed one Mantis, whom I later trapped.