Plants may improve water quality by taking in nitrates and other nutrients released when organic waste breaks down in the water. This helps keep algae at bay. Plants are great for a breeding tank because they make fish more likely to reproduce and protect fish fry as they grow. Plants help oxygenate water by making oxygen and taking in CO2 as part of the photosynthesis process. The plants give the fish oxygen, and the fish exhale CO2, which the plants use to make more oxygen.