In the presence of sunlight, algae produce oxygen through photosynthesis and release this oxygen into the pond water. At night and on very cloudy days, algae and submerged plants remove oxygen from the water for respiration. This is true for the water hyacinth as well. This leads to depleted levels of dissolved oxygen in the water bodies and can affect aquatic life.
Water hyacinths contribute greatly to this because of their fast growing property. It grows so quickly that it could theoretically, cover the whole globe in two and a half years, given the chance. They grow really fast and form thick mats on the surface of water bodies. These plants then result in depletion of the dissolved oxygen levels in the water.