Pond water turns green when there is too much single-cell algae. This is caused by an imbalance within the pond water, from too many nutrients or not enough good bacteria in the water.
Too many nutrients can be coming from fallen leaves or dead plants, fish waste, fertilizer, or sludge within the pond. A lack of good bacteria can cause a lot of problems. Good bacteria breaks down excess plant matter, fish waste, and sludge within a pond.
Depending on the size of the pond, one of the easiest ways to rid the pond of excess algae is to clean out dead plant foliage, leaves, and fish waste by using a net or pond skimmer. Replacing all or most of the water in the pond will make an immediate improvement.
Ultraviolet light filtration will kill bacteria and free-floating algae, but will not get rid of string algae or other forms of growth. These will have to be removed by hand. Although these lights can be handy, and can be purchased as an attachment to your pond filter, these lights can kill a lot of good bacteria, giving your pond the opposite problem: not enough oxygen or plant life.
Anti-algae products, although effective, will kill all the algae in your pond. Believe it or not, this is not good for your pond! Algae on the sides of the pond are signs of a healthy pond. Killing all of the algae in your pond can cause more problems, like lowering the amount of oxygen in the water or killing wildlife that lives your pond, such as fish or frogs.
To prevent excess algae and that green color from coming back, you can install a pond filter. If the pond already has a filter, it might not be big enough for the pond.
Pond filters should be large enough to filter all of the water in your pond within 3 hours. For extra-clear ponds, or those that are in direct sunlight, install slightly larger filters that will filter the water every 2 hours.
For more natural algae prevention, floating plants are a great option. Plants such as water hyacinth and water lilies will shade parts of the pool, which will hinder excess algae growth. Ponds exposed to direct sunlight will need about half of the pond’s surface covered in water plants.
Adding some barley straw to the bottom of the pond will prevent extra algae from growing. However, the barley will break down eventually, and more will have to be added.
Beneficial bacteria products are an eco-friendly option for stubborn algae growth. These products lower the nutrient levels in the pond, balancing the water and reducing the amount of algae. Much safer than chemical options, these products should not harm any wildlife that may drink or swim in the water.
Another option to keep that water extra clear is to change about one quarter of your pond water. The water drained from the pond can be used to water the lawn, garden, or bushes in the yard, and the pond can be refilled with water from a garden hose.