If you live in an old apartment or condominium complex of fewer than 10 units, you might want to check the tap water. Pour some into a clear glass and take a whiff. Does it smell of chlorine? If it does, you don't have too much to worry about. It might not taste good, but at least the chemical smell indicates it's bacteria-free. But if it smells of something else, such as mold or ammonia, or if it's cloudy or has particles floating in it, you'd rightly be suspicious of its quality.
So then what should you do? Call the local waterworks bureau? Perhaps, as these bodies are responsible for problems regarding water supply. However, if the cause of the problem is your housing facility's water tank, you're wasting your time calling municipal authorities, as in such cases responsibility rests with the owner of the building.
Unlike in one- or two-story houses, where tap water comes directly from the distribution pipes, in most buildings three or more stories high, water is temporarily stored in water tanks. This is because the pressure from the distribution pipes isn't sufficient to boost the supply up to the higher floors directly.