The most common reason a pond turns green is due to algae. First of all, algae in a pond is not necessarily a bad thing. On the contrary, a healthy thin layer of algae growing on the pond’s inner surfaces is an integral part of a healthy pond. It can help prevent a high level of nitrates from building up in the pond water by consuming the nitrates as food and giving off oxygen. On the other hand, algae can still be a problem.

The two most common problems associated with algae in a pond are:

  • When you have an algae “bloom,” and it clouds the water. This can range from a slightly cloudy green color to the point where it’s difficult to see into the water.
  • When you get the “bad” type of algae, which is referred to as filamentous algae. This type of algae does not necessarily affect the clarity of the water. Still, it can grow in clumps or stringy masses to the point that it threatens to overtake the pond.

Let us discuss these one at a time.

In the first situation, your pond water, which was once clear, has suddenly turned green. Here is what has happened. The “good or healthy” type of algae begins to reproduce at a faster than usual rate, also known as a bloom. Similar to how a flower might bloom and give off pollen, this type of algae will suddenly release millions of single-cell algae, only 3 microns in size. These cells, while they are alive, are too small for any filter to pick up.

How do you control algae blooms? The easiest and often-times most practical way to solve this problem is to install an ultraviolet light on your pond’s filtration system. The way an ultraviolet light works is simple. As the water from the pond flows through the ultraviolet light unit, it is exposed to the light waves emitted from the light inside the unit. These light waves are so intense that they break up the DNA in the algae’s cell, thereby killing the algae. Once the algae dies, the dead cells start to decompose and begin sticking to one another. Eventually, their total mass becomes large enough for the filter to remove them from the water, or they sink to the pond bottom. Once you install an ultraviolet light, this process can take about a week before the water becomes clear again.

In the second situation, your pond water may be clear, but stringy green algae reaching 1 to 2 feet in length is rapidly beginning to grow on the waterfall or throughout the pond. These algae may start to become a problem as it breaks loose and begins floating around in the pond. Additionally, as this type of algae fills the pump or skimmer basket, it may need to be cleaned almost daily. If this goes unchecked, the pump will slow down or completely stop. In this case, the problem is that you have developed filamentous algae in your pond. This algae is most commonly introduced to a pond when you add plants. If a single spore is on a plant when you place it in your pond, you can create this problem.

This “string algae” as it is often referred to comes in many forms. Some grow very quickly and are invasive, while others grow very slowly. Some forms may not be evident at first but will grow to about 2 to 3 inches long on the pond walls. With this form of algae, you notice the pump or skimmer baskets quickly get lined with green algae to the point that it affects the water flow of the pump. It is impossible for the “good” algae mentioned in the first situation to plug a basket due to its small particle size.

How do you control filamentous algae? An ultraviolet light will have minimal effect on filamentous algae. This is because this type of algae grows on the pond walls and will never reach the UV light. In mild cases, you can collect the algae off the waterfall or rocks by hand when it becomes unattractive. In severe cases, when it becomes very invasive and starts plugging the basket, we recommend using an algaecide. We have a product called Algae Control, which has worked very well for us. It is a herbicide, which means it is a weed killer. When dosed properly, it is extremely effective. But you need to know how many gallons you have in your pond to use it properly. If you overdose by as little as 20%, you can kill your fish. We suggest you under dose the first couple times you use it if you are not sure of the volume of your pond. One or two normal doses will usually kill off the algae. Then we recommend adding half the recommended dose once every two weeks, rather than once a week, for maintenance. Keep your pump running to help provide oxygen to the pond while treating with this product. As the algae dies and begins to decompose, it can consume large amounts of oxygen from the water. You need to pay attention when you use this product. Still, it is by far the easiest and most effective product we have used to overcome filamentous algae.

Tips concerning ultraviolet lights.

  • Make sure the UV light you select is adequately sized for your pond. A light that is too small for your pond will not effectively clear your pond water.
  • Make sure you change the UV lamp once a year. After one year, most lamps are operating at only about 60% of their capacity. The lamp might be lit, but it will not have the ability to kill the algae.
  • Make sure you do not exceed the recommended flow of water through the unit. If you send the pond water through too fast, it will reduce the exposure time to the light, thereby reducing the ability of the light to kill the algae. You may need to install a bypass on the unit. This will allow you to control how much water is traveling through the unit. The proper water flow is very important.
  • Make sure you have proper circulation in your pond. Is all the pond water circulating together? If you have dead areas in your pond where the water seldom moves, those areas will tend to grow algae, and as this water slowly migrates into the pond, it will cloud the rest of the water.
  • Make sure you operate your filter system 24 hours a day. If you are in the habit of running the only part of the time, you may find keeping the water clear to be a problem.
  • A properly sized UV light coupled with a good filter will give you clear water all year long with no effort on your part other than changing the lamp once a year. They work so well that we would never build a pond without installing an ultraviolet light.

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