There are many things in river and ocean water, including visible and invisible forms of contamination. There are a number of signs helpful in discovering how polluted water is, which can also indicate other properties.

• The first sign of cleanliness is the pH value. This indicator tells how acidic water is. Fish cannot live in water if it is too acidic or too alkaline.
• Another indicator is electrical conductivity. This reflects the amounts of foreign substances in water. Foreign substances may be safely left in the water as long as they are useful for marine life (such as planktons, for example), although not everything in water is harmless.
• The turbidity of water is another way to gauge pollution.
• The amount of dissolved oxygen is also important. Fish need oxygen in water to breathe. They will have a hard time if there is not enough oxygen-which sometimes happens when there is an abnormal increase in the amount of plankton.
• The water temperature and the amount of salt dissolved in water both help show pollution levels. When the electrical conductivity of ocean water is examined, the salt is counted as a part of the impurities. Therefore, it is necessary to know in advance how much salt is dissolved in water, to be able to measure other impurities accurately.

The six indicators above are essential measurements to maintain the quality of water. The water quality monitoring system measures these six criteria simultaneously, and is widely used to check the quality of rivers and oceans. It is also used for wastewater from factories, for aquariums, and for fish farms.