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Controlling algae in water gardens and ponds

Controlling algae in water gardens and ponds

The number-one question that pond-keepers and water gardeners ask is how to control algae growth. Everyone strives to have clear water so that they can enjoy beautiful goldfish, koi, and colorful aquatic plants. A pond with green water and fuzzy algae is unattractive and makes the pond look like a mess! Stringy green algae can even clog the pump and reduce the aeration necessary for keeping the fish alive. Is it possible to have a clear pond? Yes! But the first step in eliminating algae problems is understanding their biology and importance in your pond’s ecosystem. 

Algae is not always bad

Algae are an essential part of the pond’s food chain and biological balance. In most ponds, algae growth is minimal and doesn’t cause any visible problems. However, under certain conditions algae growth gets “switched on”, which results in rapid growth. We’ll never have an algae-free pond, but we can take steps to keep algae under control. The idea is to maintain the right natural balance so that it cannot take over and cause a mess. We’ll look at effective ways to tip the ecological balance away from algae blooms and toward a beautiful water garden or koi pond that stays clear and free of algae problems.

Can algae be eliminated?

The fact is, algae play an important role in your pond’s ecosystem and are necessary for a natural balance. Algae are just plants. Like other plants, they require sunlight and nutrients like phosphate, calcium, and zinc. There are two categories of pond algae. Single-celled algae cause the “pea soup” blooms that turn the water green. Even though your pond looks clear, the water actually contains millions of floating algae cells. You can’t see them because there’s not enough to color the water. Green water indicates that the algae have reproduced so much that they can be seen in the pond water. Filamentous or “string” algae look like long, green strands that grow on the sides of the pond and on waterfalls. Sometimes this type of algae forms a floating mat. The important thing to remember is that algae can’t be eliminated, but only brought under control. 

The important thing to remember is that algae can’t be eliminated, but only brought under control.

Where do pond algae come from?

When you first fill your pond with water, it looks clear like a swimming pool. That’s because the pond was not biologically mature. Nearly all of the aquatic pond life in your pond is microscopic. These invisible microbes power the biological balance and purify the water. They make it possible to maintain life in your pond and water garden. Many new water gardeners want their pond to look like a clear swimming pool, but this is not possible. Within a few days of filling your pond with water, microscopic life will make its home in the pond. Beneficial bacteria and fungi enter the pond from nearby soil and pond plants and on the surface of fish. Algae get into the pond in the same ways. 

Why are natural ponds clear?

In natural ponds, algae are kept under control by predators like algae-eating plankton, snails, freshwater clams, and fish. Unlike man-made ponds, the water is normally quite low in nutrients, which limits algae growth. Natural ponds are deeper than most water gardens. Sunlight doesn’t reach the depths, so most of the water remains clear. Some ponds are spring-fed. The constant flow of water flushes the pond of algae-promoting nutrients. 

How algae take over a water garden

Remember, algae like light and nutrients. Your backyard pond is the ideal environment for algae. The water is shallow, which allows sunlight to reach from top to bottom. Shallow ponds get warmer, which speeds up algae reproduction. Backyard ponds full of goldfish and koi have a much higher level of nutrients compared to streams and lakes. The nutrient levels can be hundreds or thousands of times higher than natural waters. Fish food ingredients contain the same nutrients, like phosphorous, that algae need to grow. Fish only digest a portion of the food they eat. Fish waste contains unused nutrients, which dissolve into the water. These nutrients stimulate algae growth, causing green water and stringy algae growth. All these factors cause an imbalance that favors algae growth. 

Tipping the balance toward clear water

While you can’t, and shouldn’t, try to completely eliminate algae, it is possible to make it harder for the algae to thrive. The following tips are proven ways of keeping your pond clear with very little algae.

  • Use floating plants like water lettuce and water hyacinth. These plants remove algae-promoting nutrients from the water and reduce light penetration.
  • Don’t overstock the pond with fish. More fish means more feeding, which adds nutrients to the water.
  • When you feed the fish, give them small amounts of food. Only feed what the fish will eat in about five minutes. Never let uneaten pellets remain in the pond.
  • Once a year remove sludge from the bottom of the pond. Remove dead plant leaves before they decay. Organic debris will release nutrients as it decomposes.
  • Keep the pond filter clean. Pond filters collect organic debris. If it is allowed to decompose, the debris will release nutrients into the water.

Other steps to reduce algae growth

New water gardeners sometimes wonder why they need a water pump and a filter for their pond. Man-made water gardens are ecologically very different to natural ponds. Technology helps us compensate for the stocking density of fish and plants, the higher nutrient levels, and the shallow depth of backyard ponds and water gardens. Here’s how technology improves the pond habitat.

Using pond treatments to control algae

There are two categories of algae control chemicals: natural and man-made. Algae-inhibiting barley straw is a slow-acting treatment that is popular with water gardeners. Bacteria products break down sludge and reduce nutrients like phosphate. Neither treatment directly kills algae. They inhibit algae growth by improving conditions in the pond. Chemical algaecides work by damaging the algae cells, causing them to die. Here’s a list of pond treatments and how they work.

  • Bacterial additives remove nutrients from the water and starve the algae.
  • Water clarifying treatments cause suspended algae to clump together and get captured by the filter.
  • Dyes color the water blue, reducing the light entering the pond.


Algae is an important part of your pond’s natural balance. You should not expect to have an algae-free pond. It can’t be eliminated, but you can control it through these proven algae-reducing methods. If you’re a water gardener, add lots of plants to remove nutrients from the water. Don’t overstock koi ponds and water gardens with fish. Keep the filter working efficiently by cleaning it every month. Natural and chemical pond treatments can help, but the best approach is proper care of the pond.

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