General Algae Control

Algae can be controlled chemically or naturally.
Very often a combination of the two methods is used. If you are maintaining fish and/or plants, your ultimate goal is natural balance. Every pond is an environment unto itself, but if you understand a few guidelines you will have clear water and controllable filamentous algae.

Chemical control usually means the use of algaecides, a chemical which kills or inhibits all types of algae to a greater or lessor degree. Such chemicals would include products containing simazine, chelated copper, or potassium permanganate. Chelated copper does not harm vascular plants but may stunt hyacinths or plants which gain nutrients directly from the water. Caution must be exercised with these chemicals as overdosing can kill fish. Simazine and copper will inhibit most water plants. All of these chemicals work best as a preventative. A massive die-off of algae creates an ammonia increase and oxygen depletion as the algae starts to decompose. This also can result in dead fish. Use these algaecides before your pond is green. Successive treatments may be necessary.





If your pond has planktonic (Pea Soup) algae, use water clarifying chemicals to clear your pond without harming fish or plants. Examples of these highly recommended chemicals are Rapid Clear, Microblift, and Pond Clear powder. Rapid Clear flocculates the planktonic algae into larger particles which settle to the bottom or collect in the filter. This flocculated material must be removed from the system or it will dissolve and become nutrients which feed a new plankton bloom. The Pond Clear powder will precipitate nutrients in the pond and starve the algae. Both of these products will help achieve a natural balance more quickly.

Natural balance is the goal

Basically, to control algae naturally we must limit sunlight and nutrients. Algae requires sunlight. Shallow ponds (less than 18″) can be difficult since the sunlight reaches the bottom so easily. Deep ponds are preferable in well lit gardens. Sheltering your pond from sunlight will make algae growth much easier to control. An overhead structure will provide shade in Koi ponds. Use 70% to 80% shade cloth and cover the entire pond. Water Lilies provide shade in water gardens. Try to achieve 70% plant coverage over surface of pond.

Nutrients are fish waste, ammonia, fish food, decaying algae, dead fish, leaves, debris, and even phosphates or nitrates in tap water. A good rain can leach fertilizer into a pond. All these sources of nutrients need to be removed from the pond or you will have algae problems. The pond must have good circulation throughout the pond to sweep debris to the drains. Excess debris on pond bottom can cause string algae. Ideally, the large particles should be removed before they decay. Use a net or vacuum. Install a settling tank or a good mechanical filter and keep these clean. Swimming pool cartridge filters and rapid sand filters make good mechanical filters but require a large pump. These filters also need extensive cleaning. Use one of the excellent prefab prefilters on the koi market. These are designed for ponds not swimming pools. Debris trapped in a mechanical filter will dissolve and leach into the pond as algae nutrients. Manually stir your rapid sand filter before backwashing.

The dissolved nutrients such as ammonia, nitrate, and phosphate need to be removed biologically. In a water garden, the aquatic plants compete for nutrients and will discourage planktonic pea soup and hair algae. In a koi pond the biological filter provides a large surface area for beneficial bacteria to grow and compete for dissolved nutrients. Biofilters should be as large as possible. The bacteria can take 2-3 months to establish. The bacteria does not grow well when the water temperature drops below 60º F. Use algaecides or water clarifiers until your biofilter is established. The biofilter will not function properly when clogged with debris. Upflow filters will need to be cleaned down to the bottom screen to loosen clogged material. A blower assembly may assist in backwashing upflow or downflow filters that use gravel. Synthetic filter medias are easier to clean manually. Backwash your biofilter with pond water to remove accumulated organics. The chlorine in tap water will kill the bacteria you are trying to grow. Use Nitrifying Bacteria to seed biofilter and Superbugs to remove sludge, help clear green water and dirty filters.

A short smooth growth of algae on the walls of a pond is essential to really establish a nice balance and helps to keep the water clear. The algae and bacteria on the walls of the pond also act as a biofilter. Try to cultivate this growth. Do not scrape the walls of your pond. Use water clarifiers or algaecides when necessary to keep the water clear so sunlight can penetrate and grow this smooth algae. This algae can grow into long filaments if the pond is out of balance with too much sun and waste products. String algae will remove fish waste when the biofilter is not functioning properly. Remove excessive growth by hand and continue following above guidelines. Springtime often causes a pond to be unbalanced. The aquatic plants and microorganisms have been dormant all winter and the warming season allows the algae to flourish. A pond overcrowded with fish will take longer to balance. Although algae control can be frustrating, don?t give up! In time you will learn what it takes to keep your own pond in balance.

Note: Rock salt @ 1 pound per 100 gallons can kill stringy algae and aquatic plants. Remove excess algae or aquatic plants before dosing with this level of salt. The decaying vegetation will pollute the pond and reduce oxygen levels. Be careful when adding salt to your pond.

Ultraviolet systems will effectively kill planktonic algae with the proper size unit for your pond. These units are quickly becoming a standard item in conjunction with an adequate filter system. An ultra violet system will speed the balance of the pond significantly.

In summary:

  • Shade the pond.
  • Remove particulate matter.
  • Use water clarifiers or algaecides to promote natural balance.
  • Cultivate smooth algae growth on pond walls.
  • Provide adequate biological competition.
  • Install a ultra violet system to control suspended planktonic algae.
  • Use rock salt @ 1#/100gal. in koi ponds only as a prevention for stringy algae.

Do not kill algae with salt as the decay process may harm your koi.

Do not use the dose of salt in a water garden.

Biofilter Requirements (bacteria):

  • Pump on 24 hours a day.
  • High oxygen concentration.
  • Slow flow rate. 1 to 2 gallons/minute/sqft surface area
  • Large biofilter surface area.
  • One pond volume turnover every 1 to 2 hours through filter.

A good mechanical filter will greatly improve the efficiency of the biofilter

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