Jeffrey St. Clair, Ph.D. wrote:
IMHO, I would just boil your rocks for about 30 minutes. Botulinum, the
bacteria which causes botulism lives adundantly in common dirt. It takes
roughly 20 minutes to kill the bacteria, so you could do 30 just for good
measure if you want. If the rocks are two big, you can bake them at 300
degrees for an hour. Does the same thing.

That'd work for me. These are no more than 6" max dia, probably closer
to 4" - the pieces of lava rock.

Also, I wouldn't recomend shale as it composed of clay and sometimes can
release petroleum into the water. Lava rocks work great. Limestone also
great. Holey Rock, which is limestone, is also a great option and really
cheap on ebay. It might raise your pH a tiny bit, but you can monitor that
pretty easily.

Ahhh, didn't think of that with the Shale. I liked it because it was
fairly thin and would make a nice platform on top of the lava as well as
a somewhat secure hiding place for the fish. Maybe we have some
limestone or sandstone around locally I can use. If not I'll likely buy
at a store. I was going to use some "Desert Rose" (Gypsum) but found
it'd likely fall apart in water over time sooo...

We have some really nice red & black granite here too and I'd thought of
using it as the gravel as it would look nice, then remembered all the
work required to crush it. Plus I had a concern with the sharp edges.
Rocks & minerals are another of my hobbies and I'm a novice at it too.

Grandpa John