In a natural setting, fish produce nutrients that are absorbed by plants, leaving very little for algae. However, many garden ponds do not possess enough plants to handle all the nutrients produced by the fish. This causes an excess buildup and produces an ideal environment for rapid algae growth.
Whether you're just beginning and want to avoid algae problems or have an existing problem to control, you’ll first want to increase the number of oxygenating plants on the surface of the pond. This is perhaps the simplest, long-term solution to keeping water clean and clear.
Floating plants, such as lilies and lotus, provide shade and reduce direct sunlight in the pond to control the growth of algae. Add submerged plants that release oxygen to the water, such as anacharis, hornwort and parrot’s feather. As a guide, one bunch of six to seven strands of oxygenating plant can be added to every two square feet of water surface, and submerged by tying to a rock or planting in a soil container.
All aquatic plants also absorb nutrients and starve the algae. After initial plant introduction, green water may occur, but will last only a short time. Established marginal plants can be planted around the periphery of the pond or in shallow sections of the pond. These are also effective in absorbing nutrients and providing shade.
One popular way to introduce plant life into the pond system without putting plants into the main pond is to construct a plant filter. A plant filter is a simple channel or small filtration pond through which water from the pond is fed at a relatively slow rate. Fast-growing plants (efficient nutrient removers) are grown within this small pond in planting baskets or are free-floating, such as water lettuce or water hyacinth. The plant filter should ideally be lined with about 2” of pea gravel, which is the best substrate to root the plants. The pea gravel catches debris and acts as a bed for beneficial bacteria. As these plants grow, they absorb nutrients from the water and “out-compete” algae to control its growth. Generally, the plant filter needs to be stocked with plants equaling approximately one-fifth the surface area of the main pond.
Water treatments are an excellent option where algae problems already exist. Green water and string algae can be controlled using repeated applications of an effective water treatment, such as AlgaeControl™ from TetraPond. Reapplication is necessary for maximum effectiveness, but be sure to follow dosage instructions.
Using a high-quality fish food will also help discourage the growth of algae, as the food will be fully digested, leaving fewer nutrients to pass through the fish.