What to do about Green Pond Water, and String Algae

Free How to Get Rid Of Pond Algae E-Book 

When it comes to getting rid of pond algae, whether it is string algae (filamentous) or green water algae (planktonic), there are several options for koi ponds, garden ponds, water gardens, and fish ponds hobbyists.


Controlling and Killing Algae in a Pond Requires Pond Balance


Balancing a koi pond, fish pond and/or water garden is done with a combination pond filtration, water quality, and pond plants such as floating pond plants, water lilies, marginal pond plants, and oxygenating pond plants. Water hyacinths and water lettuce are excellent pond plants for cleaning up ponds, They provide shade, blocking the sun, and multiply quickly. These plants can also be placed in waterfall filters to help the pond filter along. Water lilies and water lotus are also heavy feeding plants that compete with algae in the pond as well as provide shade. Oxygenating pond plants like anacharis, horwort, and parrot feather put much needed oxygen in the water and soak up pond sludge and organic debris that feeds green water algae and string algae



Algae Filters: Filtration Options for Pond Algae, Filamentous Algae,”Pea Soup” Algae



Ultraviolet Clarifier & Ionizers 

Read about How to Choose UV clarifiers here

Ultraviolet clarifiers can be stand alone units or combined with other components such as a UV Pump, all-in-one filtration system, or a UV Filter. The only sure way to eliminate green water in a garden pond, fish pond or koi pond is with an ultraviolet (UV) clarifier. It’s rumored that UV clarifiers destroy beneficial bacteria, but, it most cases, this claim is false. 

UV Clarifiers and sterilizers are designed for safe and effective plantonic or green water and pea soup algae control. Ultraviolet clarifiers are mechanical devices that eliminate green water algae for good. These UV lights are used in conjunction with a good pond filter to kill and then remove the algae from the water. There are also “uv filters” which are pond filters with integrated UV lights.

Ionizers (Ionizer, Ion-Gen, Triton Ionizer): These work excellent for controlling filamentous algae such as string algae, hair algae, blanket algae, and algae mats and/or algae growing on things in the pond. Read more about ionizers here.

Ionizers are relatively new (within the past 10 years or so) to the koi pond, fish pond, and garden pond market. They utilize a copper anode to rid the pond of filamentous algae, or string algae, and are sometimes referred to as a filamentous algae filter or string algae filter.  Ionizers can be added to almost any pond and anywhere in the pond that water moves through; i.e. inside a pond skimmer, next to a submersible pond pump, or inside a check valve assembly (back flow preventer). They work by disintegrating a copper anode thus putting a small amount of copper in the pond. Although they are marketed for green water and string algae, we have found that they work well for string algae and not so much for green water.


How to Control Algae in Garden Ponds, Koi Ponds, and Fish Ponds


Fish Safe Algae Control for Ponds

Pond algae comes in many different forms and different colors (green algae, red algae, brown algae). The two kinds of pond algae frustrating backyard pond owners most often though, are “green water algae”(also called pea soup algae) and “string algae” (also called “blanket weed” or “hair algae”).

Green pond water algae are single cell plants make the pond look like “pea soup”. These plants are so tiny that they can pass right through a pond filter. Consequently, it is necessary to use additional forms of algae control. Algaecides are a good choice in the Garden Pond or Water Garden, and tend to work well, especially when the pond has a suitable amount of aquatic plant life. Some algaecides in fish ponds and koi ponds, like algaefix, can be harmful to the pond fish if overdosed. Microbe Lift Algaway 5.4 is a very safe and forgiving algae control product for koi ponds and garden ponds that has profound results.

Controlling and getting rid of string algae and green water algae in ponds used to be difficult, but in the last few years many effective and environmentally safe products have been developed to eliminate string algae and green water algae. This makes it much easier to control and eliminate the string algae and green water algae nuisances.

Use a fish and plant safe algae control product to begin killing off the pea soup algae. If the pond is very green, use a dechlorinator and do a partial water change before adding the algaecide. Once you have noticed a significant improvement, use a “flocculant” to clump dead single cell algae particles together so they get picked up by the pond filter.

Flocculants are available that are harmless to Pond Fish and Pond Plants, but make the Algae clump together so it can be picked up by the Pond filter. It is advisable to clean the Pond filter more regularly for few days after using the product, so the green water Algae can be efficiently eliminated.

Use a fish safe oxygen cleaner to clean your waterfall, pond liner and other surfaces without scrubbing (not within a few days of using an algae control product).


Natural Pond Remedies to Balance Koi Ponds, Fish Ponds, and Garden Ponds


PondMarket products are an excellent choice for use at keeping ponds clean and clear. Algae management in ponds is a question of balance and Pondmarket products are very safe, concentrated, and very easy to use. Our Fizzy Kicks, a natural blend of beneficial bacteria with barley in tablet form, cleans up the gunkiest of ponds. There is no smell, no measuring, and no pouring. Our flocculent, Sheer Clear, is natural and does not cloud pond water or remove oxygen while working (unlike PondCare Accu Clear and other brands). Our Barley Fusion keeps nasty growth off of rocks and helps clear pond water and keeps growth off of waterfall rocks, and pond rocks. These products are natural, concentrated, and developed by hobbyists for hobbyists.


Balancing the pH


Test the the pH and adjust the pH up or down so the pH test reads between 7.0 and 8.0 (always measure pH in the morning, it goes up during the day).

It is very important to balance the pH in the Garden Pond. Although pond fish tend to not be effected in high pH, nothing in the pond will be very successful until the pH is balanced. Easy to use pond test kits are available to measure the pH. There are digital test kits and Wifi Pond Monitoring Systems available for those of use that enjoy quick and easy solutions. Pond Algae in its many forms and pH are inter-related. The higher the pH, the more algae that grows in the water garden, fish pond, or koi pond.

Read about Pond Water Quality here.

Once the pH is balanced you can introduce beneficial pond bacteria (read about beneficial pond bacteria) and lots of aquatic plants to compete with the Algae for nutrients and hopefully starve the green water Algae.

String algae (or blanket weed or hair algae) often grows in water with high pH values. The pH must be lowered to 7.0 – 8.0 to slow the growth of this type of algae. There are many products available to eliminate existing string algae safely without harming the aquatic plants in the pond. Oxygen cleaners are best to clean existing stringy growth on waterfalls, pond liners and other pond surfaces without scrubbing, Barley products will help prevent future string algae growth.


Where Does pond Algae, Pea Soup, and String Algae Come From?

Pea soup algae and string algae are a result of organic debris from fish waste, decaying plant material, debris lodged in gravel in the bottom of the garden pond, and sunlight.

If the pond has a gravel bottom, ensure that there is not a lot of organic debris in the gravel. If a pond clean out is not practical, use PondMarket Fizzy Kicks or Microbe Lift Sludge Away to eat up the debris locked in the gravel. Left untreated, nothing will clear the pond up.

Add aquatic pond plants to shade the pond water and compete with algae for nutrients.

Be sure to remove organic debris on the pond bottom with beneficial bacteria, a pond net, and/or a pond vacuum.


Removing Phosphate Removes Pond Algae


Test phosphate levels and use a phosphate remover if necessary.

Phosphate is pure fertilizer. Koi ponds, garden ponds, and fish ponds that have habitual algae problems typically have elevated phosphate levels. When phosphate levels are elevated, algae controls can be added for weeks with no apparent affect. Elevated phosphate levels are a result of overfeeding koi and goldfish, feeding a substandard pond fish food, using too much aquatic plant fertilizer, a pond plant fertilizer that has phosphate in it, and/or an over abundance of organic debris or sludge in the pond. Elevated phosphate levels is also inherent in new ponds or freshly cleaned ponds because it is a byproduct of an inefficient nitrogen cycle. Phosphate levels can be tested using a phosphate test kit. If the pond needs to be treated, a fish and plant safe phosphate remover can be used and is recommended for use with beneficial bacteria.

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