Do you have a nitrate test? Take the pagoda stone out of the tank and put a drop of #1 nitrate test on it. If it gets bubbly you have your answer, it is the culprit. If you put the stone back into the tank be sure to rinse and rinse and rinse it! Go ahead and test your sand as well but that isn't likely to be the problem as sand that dissolves wouldn't be very good filter media.