Why would I put it in the rock? I am talking about adding it to the change out water.
On another subject here is a quip about Thorite concrete having a low ph.
http://www.thekrib.com/TankHardware/concrete.html
"When I was General Curator at the Mystic Marinelife Aquarium
in Connecticut, we used a cement mix called Thorite, which is
made by Thoroseal, to fasten pieces of living coral to substrate
before submerging it into our displays. It sets fairly quickly
and can finish curing underwater. We found no effects on pH
when we used it. To test its use in fresh water, just make up
a glob of it, let it set a while, and place it in a jar of fresh
water to finish curing. Test the pH before and after and see
how it looks. If no changes are noted it shouold be safe.
I don't know what effect it might have on hardness in fresh water
but you can test it out the same way and see what happens.
We
We used it a lot and had no real problems with it.
Got to go now. Good luck.
"