Lately our toilet bowl has started being bit slimy (even regular cleaning and Clorox bleach toilet tabs in the tank is not helping) and there is black build up below the water, along with the water coming out of the tab feeling greasy/oily. I have checked with couple of neighbors and they do not seem to have the problem that we are having. Any idea what I am potentially facing here? I have someone coming to test the water for hardness tomorrow but anything else I should be explore?