Ozone, O³ or trioxygen, is a tri-atomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen which is much less stable than the di-atomic O².
Ground-level ozone is classed as an air pollutant by the W.H.O. with harmful effects on the respiratory systems of animals however, the ozone layer in the upper atmosphere filters potentially damaging ultraviolet light from reaching the Earth's surface and helping protect us from skin cancer. It is present in low concentrations throughout the Earth's atmosphere. Properly controlled, it has many industrial and consumer applications.
Ozone has been used for decades in the treatment of drinking water, swimming pools and marine aquariums. In more recent times its use has become more popular in Koi ponds as people have come to realise the benefits. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water.
It works because when Oxygen is converted to Ozone it becomes a 3 Atom Molecule instead of 2. This third Atom makes the Molecule very unstable and it immediately wants to separate again. When it does so, it oxidises (Incinerates) any organic matter it comes into contact with such as bacteria, algae etc. When the unstable atom has burned off, what remains is pure Oxygen.