Texas Holy Rock

I'll frankly say my thoughts on the nitrogen cycle are debatable. But certainly not harmful.

Any rock that came from a FW tank I would first nuke with bleach (3 days in a bleach solution, 3 days in a RO bath, dry out for a long time). Once the rock is thoroughly dried (weeks really), it will render any left over chlorine inert. Just to sanitize it of any thing dead in there.

Past that, there are certain rocks you definitely don't want in SW that are fine in FW, but THR is not one of them. If you've treated the FW tank with copper, it may be the case that any rock is useless for SW now. But if you never treated the FW tank, then you should be fine in that respect.