Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Yes, take out the white rocks and test them in a bucket or glass of tap water.
Test the pH when you start, then test the pH a few days later.
Often white or off white rocks are limestone or related and are pretty much calcium carbonate or magnesium carbonate.
Calcium and magnesium are measured as GH, and this is the parameter that is most important to the livestock.
Carbonate and bicarbonate are measured as KH, and these act as a buffer to stabilize the pH. In general, high KH means high pH that is difficult to change. Anything you add to lower the pH may work for a short time, but then the KH makes the pH rise again. Low KH may mean low pH, but it really means the pH will be controlled by something else.