I am pretty sure it is not a sulphur smell, and it does not seem like rotten eggs either. It is a gaseous odor, and it almost seems to be coming from under the rim. The tank water does not stink. When the toilet is used for a bm, the two “gaseous” odors make a poisonous stench. My parents are on the same water system and we do not notice anything when we use THEIR toilet. I have dumped bleach down the overflow to clean the rim out, and that did not work. Since the tank water and my parents toilet are ok, I guess it is not the new water system. What on earth is causing this?