They provide oxygen during the day, but like any other plant, they use oxygen during the night. Their main function in the pond is cooling the water by shading. They are planted in gravel instead of soil to force them to extract nutrients from the water that would otherwise feed algae growth. These plants make a great spawning media for goldfish, giving a place for the fish to deposit eggs for fertilization and a place for their baby fish to have protection from being eaten by the adults. Submerged plants are not fertilized at all.
Water lilies are heavy feeders for the best foliage and flower production.
They are fed according to label instructions either monthly or once in spring and once in summer if a slow release fertilizer is used. Water lilies are fertilized when the first floating leaves appear in spring. Bog plants are fed when they have started growing actively. Lotuses are given their tablets once the first leaves reach the surface of the water. The tablet is inserted into the soil, finger-deep approximately three inches from the growing points of the plants.