How to control algae and green water in your pond

Algae—it’s the number one complaint of pond keepers. This ubiquitous, unwelcome plant life in all its green glory is the bane of the pond keeper's existence and can make the simple pleasures of pond and fish keeping seem like chores. In addition to other non-life-threatening challenges, algae obscure colorful fish and deplete valuable oxygen. The good news is, with a few simple steps, you can stop seeing green and start seeing the beautiful, unobstructed tranquility of your water garden.

First, understand it.
Algae are primitive plants that, via photosynthesis, combine water and carbon dioxide to form sugars for energy and growth. Algae produce oxygen, a useful by-product, but when sunlight is not available at night, they quickly respire. This respiration uses the stored sugars and oxygen to form carbon dioxide, which depletes the oxygen in the pond. There are basically two types of pond algae:
  1. Green Water: These single-celled organisms—which remain suspended in water—are so tiny, they pass through even the finest filter. If conditions are right, meaning there’s plenty of nutrients and sunlight, as many as five million algae cells per milliliter of pond water can be present.
  2. String Algae (also known as “hair algae”): This filamentous species, which grows in long strands, adheres to rocks and waterfalls. They eventually tangle together, forming thick, unsightly mats that can double their weight within 24 hours.

Then, eliminate it.
The following are some tried-and-true methods that will not only help you treat algae, but also help prevent it.

Add Plants
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
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.

Fish Feeding
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.
Green Water Control: Ultraviolet (UV) Clarifiers
UV clarifiers combat green water by exposing suspended single-celled algae to very high levels of ultra violet light, which destroys its reproductive ability. UV clarifier units consist of a tubular fluorescent bulb that emits UV light. Because UV light is harmful to the human eye, the bulb is enclosed in dark, opaque housing.
Here’s how they work: Pond water enters through the clarifier’s inlet tube and travels around the UV light. The UV light kills the suspended algae, causing them to clump together into particles large enough to be removed by filtration, and then exits the clarifier. Finally, impurities are removed from the water as it passes through a mechanical and/or a biological filter, exiting back into the pond.

String Algae Control: Garden Hose, Hand, or Net
Since UV clarifiers are ineffective against string algae, pond keepers either use a garden hose to blast it off rocks and waterfalls, or remove it by hand or net.

When it comes to controlling algae, balance is best.
No pond is ever totally free of algae, but in a balanced environment, algae can be kept in check. Understanding how it grows is a good start, followed by an appropriate treatment for the type of algae present. UV clarifiers, water treatments, and other algae eliminators are effective methods for treating and preventing algae proliferation. But don’t ignore Mother Nature; the addition of plants should be part of the long-term solution.
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