Dissolved oxygen (DO) meters are used to measure the amount of dissolved oxygen in a liquid.  Oxygen makes its way into water through a variety of processes, including aeration, as a byproduct of photosynthesis, and from the surrounding air. Any healthy aquatic system which can support aerobic life must contain certain amounts of oxygen; healthy bodies of water typically register at least 5 mg/L of dissolved oxygen. DO levels less than 5 mg/L can result in stressed aquatic organisms, and levels that remain as low as 1 mg/L for even a few hours can result in widespread fish kills. Conversely, dissolved oxygen saturation can also be harmful to aquatic life.