From The Aquarium Wiki
Zinc (Zn) is a metal. It is an essential element, necessary for sustaining all life.
Zinc is an activator of certain enzymes, such as carbonic anhydrase. Carbonic anhydrase is important in the transport of carbon dioxide in vertebrate blood. It is also required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and anaerobic respiration (alcoholic fermentation).
Zinc is considered a micronutrient for plants.
Zinc is generally not required to be specifically added to an aquarium as it is naturally present in soil and fish food. However Zinc in large amounts is toxic to life, especially invertebrates.
It is often found in large amounts when used in non-toxic Lead weights.