Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Take the rock out and test it in a bucket of water.
Put it in pure tap water, test GH, KH, pH (and TDS if you have a meter) when you start, then test the next day, then every few days for a week.
That rock sure looks like the culprit to me!
However, test a handful or two of gravel at the same time. (separate container, of course).
If you have any of the potting soil left over, test that too. (in a 3rd container) Some potting soils have calcium carbonate in some form added to keep the pH from dropping too low.
Might be beneficial to test pure tap water with nothing in it, too. (4th container)