Abstract: A system for creating liquid gradients is disclosed, comprising at least two containers that can be manufactured separately, such as by molding plastic. Each container has an outlet near the bottom and an air vent near the top, and the outlets are connected via tubing to a receiving device in which the liquids are mixed. One container has a horizontal cross-section which decreases as a function of vertical position between the top and bottom of the container; the other container has a horizontal cross-section which increases vertically in a complementary manner. The containers preferably are held together by a rack or other means. As the liquids drain from both containers simultaneously, they remain in hydrostatic balance with each other.