It shouldn't need curing, but you can put it is a tupperware container for a few days with a PH
and a heater to see if you get a spike. If you don't then go ahead and add.
As for the mantis, you could always do a SW
soda water dip for 1 minute. Simply get a bucket, fill it with soda water (make sure it is pure soda water). Add salt mix to correct spg and place the rock in it for 1 minute. Being salt water, it should not kill off much of the good stuff, but the critter will come leaping off. You them pick out the critter you want, rinse the rock in another bucket with normal salt mix and place it in the curing container or you tank.
It works pretty well and sure beats a FW
Or better yet, skip the soda water dip all together and trap the unwanted critters as you find them. With #20, you may not even get any.