University of Illinois Extension
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Water Gardening
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Algae Problems in Water Gardens

Algae problems in water gardens are generally caused by the action of the sun and excess nutrients in the water. If ponds are established properly and balanced with the correct ratio of plants, fish and scavengers, algae control and algae problems are not difficult to control. When first establishing a water garden, green water conditions will exist for about 2-3 weeks but will clear as the water equalizes between plant nutrient uptake and the introduction of nutrients from fertilizers and/or fish. The general cause of algae in ponds is excess nutrients from either the overstocking or overfeeding of fish, overfertilization of plants, or the lack of nutrient uptake by plants caused by planting too few plants in the pond. A general rule for stocking fishponds is to use 4-6 goldfish and one water lily for every square yard of water surface. Feed fish a specified amount of food per day, and only if necessary. Most fish can survive on naturally occurring foods within a balanced system.

General methods for reducing algae in ponds include the reduction of nutrients causing the algae. To reduce nutrients, reduce the feeding of the fish, the level of fish stocked in the pond or the fertilization of the plants. Other methods to reduce algae problems are flushing the pond and adding new water; adding additional aquatic plants to allow as much as 50 percent of the water surface to be covered; and adding some type of mechanical or biological filter system in larger ponds.

The use of algae reducing chemicals should be the last resort in small ponds. Injury to both plant and animal life in the pond can easily result with misapplication. Copper compounds are the most often used but must be used in strict accordance with label directions. It is often better to look at the cause of imbalance and correct it before resorting to chemicals.

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