What are you using for a substrate, CC
You can add LR
, but I would cure it in a separate container (rubbermaid or the like) before adding to to the tank to prevent a new cycle and the ensuing stress to the existing livestock.
How bid is the tank?
If you carefully place the rock there should be little danger of it moving around or scratching your tank.
What livestock do you have in the tank?
is going to be large enough and weigh enough that there is little danger of inhabitant moving it around. I have some smaller pieces in my tank for a pod pile that the crabs and brittle stars manage to rearrange a bit