Is sandstone the problem?

Question
I bought a sandstone centerpiece at Petco for my 10 gallon aquarium (rainbow colored stone). Is that okay? The pH has been dropping to about 6.2.  When I change half the water and put in a scoop of "proper pH 7.5" buffer salts, the pH is back to 6.2 within a couple of days. We are worried because the larger fish (4 inch body, large fan tail, 4 years old) has been acting sick -- tail drooping badly, very lethargic and does not seem to be responding to the usual things that work (water changes, feeding peas). I am wondering if the sandstone could be the problem. Thanks!

Answer
Yochanan,                                                    If you add someting that has changed things and take it out... sand stone absorbs mineral.......... sodium bicarb is  mineral. It is just baking soda....which is your ph buffer ... ...do not let your fishy suffer ,............ Good Luck , Tina