Aquatic plants, also known as water plants, refer to herbaceous plants that can grow in water. They provide habitat and shelter for many aquatic animals and serve as food for animals like snails and waterfowl. Keeping aquatic plants in fish tanks helps increase the oxygen content in the water, allowing the fish to get more oxygen. Additionally, aquatic plants absorb soluble substances from animal waste, preventing water quality deterioration and making the water appear clearer.