The older toilets were designed to work with more water. If you simply reduce the water volume you may end up flushing 3 or 4 times to clean it out. That is what happened when the law first came out. Manufacturers sold the old toilets with modified controls that supplied 1.6 gpf. The darn things didn’t work and everyone began complaining about having to flush 2 or 3 times. Most newer toilets have been re-designed to function properly on less water. Be careful however, because some toilets work a whole lot better than others with 1.6 gpf.