Certain chemicals will add oxygen to the water. Plants and animals remove it through respiration. Organic wastes which contain nitrates and phosphates stimulate excessive aquatic plant growth which will increase demand of oxygen. Anaerobic bacteria, active when excessive oxygen matter is found in the water, multiply and increase oxygen demand. Chemicals added to the water will react with the oxygen that is dissolved in it removing it from the water.
Cold water has a greater capacity of dissolving oxygen that warm water.
Any increase in water temperature will reduce the capacity for holding dissolved oxygen. The majority of fresh water is in the range of 5.5 to 10. Nitrogen is a necessary components of an environment in which excessive aquatic growth arise. Phosphorus - Essential element in the growth of plants and animals.