Controlling Algae in Ponds and Water Gardens

The number-one concern with pond-keepers and water gardeners is excess algae growth. We work hard to keep the water clear so we can enjoy our beautiful fish and colorful aquatic plants.

Green water and stringy algae are unsightly, clog the pump, and make the pond look like a mess! Is it possible to have a pond with clear water? Yes! But the first step in controlling algae is understanding their natural biology and the role in your pond’s ecosystem.

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” resulting in rapid growth.

We’ll never have an algae-free pond but we can take steps to keep it under control. The idea is to maintain the right natural balance so algae 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 truth is, algae play a key role in the aquatic food chain and are necessary for a natural balance in the pond. Think of algae as tiny plants that live in wet places. Just like other plants, algae need sunlight and nutrients like phosphate, iron and calcium.

There are two common “types” of pond algae. There’s single-cell and stringy filamentous forms.

The single-celled algae cause the green “pea soup” blooms that cloud the water. Clear pond water actually contains floating algae cells, but the single cells can only be seen with a microscope. Green water indicates the cells have reproduced so much that they cause a green tint in the pond water.

Filamentous algae attach to the sides of the pond, plants and even on waterfalls. Some forms of string algae form floating matts. Sheets of blue-green “Slime algae” may develop on the water surface. This type of algae is really photosynthetic bacteria, not a plant.

The important thing to remember is that algae are not enemies to be eradicated, just brought under control. That is a good thing since it is impossible to eliminate all algae from our ponds. We just have to know how to reduce the chances of stimulating a bloom.

Where do algae come from?

If you remember when you first filled your pond with water, it looked crystal clear. That’s because the new pond was “sterile” and biologically inactive except for the plants and fish you added.

From an ecological standpoint, the new pond was more like a bath tub than a pond. Over 90% of aquatic pond life is microscopic. The biological balance that makes it possible to maintain life in your rubber-lined pond is due to billions of bacteria, fungi and plankton that purify the water.

Many new pond-keepers want their water to look like a clear swimming pool but this is not possible. Within days of adding water, your pond will become “seeded” with microscopic life. Beneficial microbes will get washed in from nearby soil, splashing water, potted and floating plants and on the surface of goldfish and koi. Algae enter the pond the same way. Nature is determined to take control of your pond!

Why are natural ponds clear?

In nature, algae growth is kept under control by natural predators like algae-eating fish, zooplankton, snails, crayfish and freshwater clams. Nutrients in the water tend to be quite low too, which limits algae growth.

Most natural ponds are deeper than backyard water gardens. Sunlight can’t penetrate most of the water, so it remains clear and algae-free. Some natural ponds also have a constant flow of water that flushes the pond and helps keep it clean and clear.

You may have noticed that algae-choked ponds tend to be stagnant and get water run-off from nearby fields. Excess nutrients and low-oxygen conditions favor algae growth, leading to stagnant water that can only support algae and mosquitoes.

Why does algae overtake a garden pond?

Remember, algae are plants. They require light and nutrients to grow. A backyard pond is the ideal environment for algae. The water is relatively shallow with plenty of light penetration. The shallow depth also allows the water to get warmer, which speed up algae metabolism and reproduction.

Man-made ponds containing goldfish and koi tend to have a much higher level of nutrients than is found in nature. Algae-promoting nutrients can be hundreds or thousands of times higher than “normal.” Koi and goldfish require feeding. Fish food contains the same nutrients algae need to grow. Fish digest only a portion of their food. Fish waste contains excess nutrients, which are released into the water. More fish means more feeding. More feeding means more nutrients added to the water.

These nutrients fuel algae growth which results in green blooms in the water and stringy algae on rocks, plants and water falls. All these factors create a biological and chemical imbalance that favor algae growth.

Tipping the balance toward clear water

While algae cannot be completely eliminated, we can reduce the chances of algae problems by making it harder for the tiny plants to grow. Here are some steps that will help keep your pond clean and clear.

  • Add plenty of floating plants like water lilies, water lettuce and water hyacinth. They reduce light penetration and remove algae-promoting nutrients from the water.
  • Feed the fish small amounts of food. Over-feeding just feeds the algae! Only feed what the fish will consume in 1 minute. Never let uneaten pellets remain in the pond.
  • Don’t add too many fish. More fish means more feeding. Over-crowding increases the chances of fish health problems and algae growth.
  • Remove sludge from the pond. Organic debris releases algae-stimulating nutrients as it decomposes.
  • Keep the filter clean. Most pond filters don’t remove algae but they do collect organic debris. Decomposing debris will release nutrients like phosphate into the water.

Using technology to reduce algae growth

Many ask why they need pumps, filters or UV sterilizers for their backyard pond.

It is important to remember that man-made ponds and water gardens are very different biologically and chemically compared to natural ponds. Technology helps counteract the unnatural stocking density of fish and plants, elevated nutrient levels and shallow depth of backyard koi ponds and water gardens.

  • Water circulation brings oxygenated water to the lower level of the pond, preventing low-oxygen conditions near the sediment. Stagnation allows debris to collect on the bottom. Under these conditions permit nutrients to be released from the sediment and into the water.
  • A properly maintained pond filter removes organic debris from the water before it breaks down and releases nutrients. Keep the filter clean!
  • An ultraviolet (UV) clarifier will kill free-floating green-water algae. UV clarifiers will not kill string algae that is attached to the pond.

Using water treatments to control algae

Algae control chemicals fall into two categories: natural and man-made.

Natural treatments include algae-inhibiting barley straw and bacteria products. Neither directly kills the algae. Rather, they inhibit growth by changing the conditions in the pond. Natural products tend to work slowly.

Chemical algaecides work faster. Neither type corrects the conditions that caused the imbalance. They can reduce the population of algae and make it easier to bring the pond back into a natural, healthy balance.

  • Chemical algaecides kill the algae by causing their cellulose cells to fall apart.
  • Bacterial additives can remove nutrients from the water and help starve the algae.
  • Water clarifying chemicals cause suspended algae to clump together and settle in the pond or filter.
  • Dyes color the water blue, shading the algae and inhibiting green water.


Algae is a natural and important part of your pond’s ecosystem. It cannot be eliminated but can be controlled through proven pond-keeping methods.

For water gardens the first step is adding plants that shade the water and remove algae-promoting nutrients from the water.

For koi ponds and water gardens, avoid over-stocking and over-feeding the fish.

A UV clarifier will eliminate murky green water algae blooms in any pond. Maintaining the filter and removing organic debris from the ponds improves water quality and removes algae-promoting nutrients from the pond.

Natural and chemical treatments help but are not a substitute for proper care of the pond.