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by Tracy Hoffman May 31, 2019 4 min read

Wth correct filtration algae can be minimised in the pond system, however it is unrealistic to expect that a pond will have no algae. While pond water should be clear and free from suspended algae, all ponds have algae to some extent on flat surfaces and this is perfectly healthy. Algae lives on excess nutrient in the pond (nitrates and phosphates). Ponds are natural systems and consequently the quantity of algae will change with the seasons. Algae will grow on all flat surfaces in contact with water, the job of fish is to keep this algae trimmed so that it does not become thick and unsightly. Floating algae should not exist in your pond. Even in a full sun position, pond water should be crystal clear. The job of a filtration system is to ensure clear water.
See our fact sheet Choosing a Filtration System:

String algae is a particularly annoying problem for the pond owner as it seems to grow better in clear water. Generally string algae appears at certain times of the year, often with the change of seasons and when other pond plants have a reduced growth phase. This is why it is important to have a mixture of plants that grow best at different times of the year as part of your pond system. The best way to minimise string algae is to increase both fish and plant stock. Increased numbers of fish will eat algae prior to it becoming long and stringy. Increased plant stock will reduce the amount of sunlight entering the pond and also remove excess nutrient from the pond system. Plants that have leaves that float on the water surface, such as water lilies are paricularly useful in reducing sunlight entering the pond.Excess nutrient can also be removed with the application of Zeolite. Zeolite binds with ammonia ultimately reducing nutrient available to algae.
Reduce fish feeding during times of algae growth, this will encourage fish to eat algae and reduce available nutrient from excess food. Ponds with too few fish, or fish that are too small, are more likely to have algae problems. Koi and Goldfish both do a good job of eating algae.
Where string algae is very long and thick fish will be unable to eat it, so it needs to be manually removed from the pond, the easiest way to do this is by twirling it around a stick.
We recommend that you do not use algaecide products to kill algae. Even all natural products weaken pond plants, in some cases killing them outright (many water lilies are highly sensitive to algaecides). These products may kill algae but they do not remove the cause of the problem, excess nitrate and phosphate. Therefore algae will return to your pond within a short period of time, growing with increased vigour as a result of weakened pond plants no longer removing the same quantities of nutrient. Worse still, the sudden death of all algae in the pond system can result in high levels of ammonia and nitrite as the algae breaks down, which may result in fish death.

Other handy hints:
  • When designing a waterwall feature choose dark or multi coloured tiles. Algae is very visible on pale coloured walls. Algae will grow on all flat surfaces including waterwall features.
  • Do not empty your pond and scrub algae off your pond walls. This upsets the natural balance of your pond system, which can take many months to recover and may lead to fish death. Ultimately the algae will return to your pond.
  • Plants do not “cause” algae, they take excess nutrient out of the pond system. However, potting plants in nutrient rich potting mix can result in excess nutrient in the pond system, resulting in algae blooms.
  • Frog ponds with small fish such as Native Pygmy Perch will have greater quantities of algae on the pond walls as these fish do not do a good job of eating pond algae. However, this is generally not a problem, it may just look a little unsightly.
  • Remove debris and sludge (leaves, rotting plant matter etc) from the pond base, this will ultimately reduce available nutrient in the pond system. Clumping algae that rises to the pond surface on warm days is caused by sludge and debris on the pond base.
  • Apply Zeolite during algae blooms to reduce growth of algae (Zeolite is a natural volcanic product that binds with ammonia, it will not kill algae outright).
  • Application of pond bacteria will assist in the breakdown of pond sludge and reduce ammonia, nitrites and phosphorous, ultimately reducing algae growth.
  • Always run your filtration system 24 hours a day.
  • Replace UV globes to UV Clarifiers every 12 months.
If you have a pond with plants and fish, you will have to deal with algae to some extent, this is normal and natural, so relax, be patient, utilise some of the above tips and remember your fish are probably not as stressed about the algae as you are!