There are literally thousands of algae varieties. However, the two that may be occurring in your pond are either single-cell algae (green water) or string algae (long, hair-like). The presence of algae does not mean your pond is unhealthy, but it can obscure your colorful fish and, if left unattended, it can grow dramatically and deplete valuable oxygen needed for the health of your fish. With a few simple steps, you can stop seeing green and start seeing the beauty of your water garden.
In a natural setting, fish produce nutrients that are absorbed by plants, leaving very little for algae. Therefore, increase the number of oxygenating plants below the surface of the pond. Add these submerged plants that release oxygen to the water, such as Anacharis and Parrots Feather. As a guide, one bunch of 6-7 strands of an oxygenating plant can be added to every 2 square feet of water surface. Floating plants, such as Water lilies, Hyacinth and Lotus, provide shade and reduce direct sunlight to control algae growth
Water treatments added to the pond water are an excellent option where algae problems already exist. There are many to choose from, so be sure to ask your local retailer to make a recommendation. Some water treatments work specifically to combat green-water algae, while others quickly clump algae so they can be removed by filtration or a mesh net. If string algae are plaguing your pond, our first recommendation is to remove as much of it as you can, using a sharp stream from a garden hose nozzle and a stiff brush. Remove string algae from the pond with a net, or use your hands.
Ultraviolet (UV) Clarifiers
UV Clarifiers combat green water by exposing suspended single-cell algae to very high levels of ultraviolet light, which destroys algae's reproductive ability. Remember, UV clarifiers are very effective against green-water algae, but they do not control the growth of string algae.