Water lilies are among the most attractive of all pond plants and relatively easy to keep. They help to maintain the water quality in the pond, because their roots absorb nitrates produced by the decomposition of fish feces. A mat of lily leaves on the pond’s surface will reduce the amount of sunlight entering the water, protecting the fish from sunburn and restricting algal growth. It will also enable the fish to dart out of sight when a predator’s shadow darkens the pond. Do not plant tropical lilies in water that is below 75°F (24°C); otherwise, they may not grow but simply remain dormant or even rot.