Fertilizers are mixtures of nutrients that are added to the soil to make plants grow better. They are used in farms, gardens and on people’s lawns. When fertilizers are overused, the nutrients can’t all be absorbed by the plants and may end up being carried into waterways. The excess nutrients can cause plants like algae to grow rapidly. Algae is important in an ecosystem, but too much algae can make oxygen levels drop because dead algae causes bacteria to thrive and these bacteria use up oxygen. Low oxygen levels can kill fish and other creatures that need oxygen in the water to live.