Why Is My Pond Green?

Why is my pond green? Is it because of algae? Algae is a microscopic plant that forms naturally in water. It will form in swimming pools, garden fish ponds, and even in dog’s bowls that are filled with water and left in a warm place.

But while it quickly becomes a problem in swimming pools, and should undoubtedly be scrubbed from the surface of animal bowls, it is not necessarily a problem if algae forms and flourishes in koi and goldfish ponds.

The real problem with algae in a fish pond is that the owner can’t see the fish even if they are gold or multi-coloured koi.

There are various different types of algae, but what they all have in common is that they make the water cloudy and they grow incredibly quickly.

Most algae are green or brown in colour. If your pond is cloudy it is almost certainly due to suspended algae.

Why humans don’t like algae

While koi and goldfish are happy to munch on algae, it spoils the aesthetic effect of the pond for us!

After all you can’t see anything very much if the water looks like pea soup, or if there is a blanket of algae weed on the surface of the pond…

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What makes algae flourish? Algae thrives in summer, especially when the days are long and hot. In the cold winter months, on the other hand, the algae will die off and the water will clear naturally. But the problem is that because they are such a simple, quick-growing primitive plants, just a few dormant cells will quickly bloom as soon as the conditions change and it becomes hot again.

Because algae flourishes in warm conditions, it also stands to reason that it will do best in shallow ponds, because the water warms up more rapidly. But even a deep fishpond can become the perfect place for algae to grow and thrive. And of course, the deeper the pond, potentially the greater the growth may be.

Of course it isn’t only heat that algae need, they also need nourishment, and this comes in the form of nutrients like nitrates and phosphates. Both of these are readily available in any fish pond water that is not carefully controlled.

Controlling algae in our fishponds

It is reasonably easy to control algae in garden ponds, depending largely on whether the water in the pond is still (with no movement), or if the water is recirculated using a pump.

Steps for still ponds include:

  • Shading the pond to keep it out of direct sunlight
  • Using chemicals, specifically algaecides that will kill off algae and blanket weed
  • Planting pond plants that take over from algae and establish themselves within the water
  • Putting bales of barley straw or oat hay (or straw) into the water that gradually breaks down, and effectively gets rid of algae
  • Adding a beneficial bacteria to the water that will get rid of the nutrients that the algae need to thrive

The algae in recirculating ponds can be controlled by installing an Ultra Violet (UV) clarifier – frequently referred to as an UVc. These units have become very popular in recent years as they are very effective and easy to install. Used in conjunction with either a biological or mechanical filter, a UVc will kill the algae (and also bacteria and various fish parasites), and clear the cloudy water in your koi pond. Alfagrog and Viresco can help this problem…

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