Water-closets within buildings shall be supplied with water from special tanks or cisterns, which shall hold not less than six gallons, when filled to the level of the overflow pipe, for each closet supplied, excepting automatic or siphon-tanks, which shall hold not less than five gallons for each closet supplied. A group of closets may be flushed from one tank, but water-closets on different floors must not be flushed from the same tank, except flushometers, volumeters, or similar devices. The water in said tanks must not be used for any other purpose.