Blanketweed or String Algae in Koi Ponds

Page Summary:  Blanketweed or string algae is a major problem for many pondkeepers. After cloudy water the presence of blanketweed in a pond is perhaps the second biggest complaint. Blanketweed is a form of algae and as its name suggests it looks and feels like a very wet blanket when removed from the surface of the pond water... herein lies the problem. Floating blanketweed is extremely unsightly and it grows at such a rate that the whole pond surface can become completely covered with this slimy, ugly mess. Its rate of growth is enormous. Clear the surface one day and its back the next. Blanketweed floats because oxygen generated by daytime photosynthesis bouys the algae strings so they float to the surface. At night the blanketweed might drop to a lower level. Blanket Weed ponds tend to be clear water ponds. To get rid of blanketweed you need to remove its food source... dissolved nitrates and to a lesser extent phosphates. I only recommend one natural UK manufactured, sold worldwide, product to resolve the problem... Viresco

Blanketweed Problems

How to rid your pond of blanketweed or string algae:

Blanketweed is a major problem for koi and goldfish ponds. Growing abundantly in ponds that are teeming with life, this green string like algae is particularly invasive in ponds, especially those that are less than well maintained. If left unchecked, it will quickly dominate the entire surface area of your pond in a matter of days or weeks, leaving your pond looking ugly, with your koi nowhere in sight.

Blanketweed floating algae is also extremely difficult to get rid of; unless you understand where it comes from and unless you can clear your pond system of nitrates and to a lesser extent phosphates dissolved in pond water. Unless the nutrients are removed permanently the blanketweed will return with a vengeance.

Causes of Blanketweed Floating Algae: In order to treat blanketweed, we must therefore understand where this green algae comes from and why it grows so rapidly.

Blanketweed is a form of algae so it responds to sunlight and food nutrients such as nitrates in solution. The only long term solution I recommend is Viresco... see above left

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Pondwater containing blanketweed tends to be very clear which exacerbates the problem since mnore sunlight is absorbed by the watery blanketweed and it continues to grow quickly.One of the biggest attractions for this floating algae is clear water. If your koi or goldfish pond is too heavily filtered, all of the organisms that would otherwise block out the sun’s rays and thus prevent photosynthesis, are removed.

This, combined with the additional warmth, provides the perfect setting for blanketweed’s floating algae to thrive.

The most crucial element that allows blanketweed to thrive is dissolved nitrate and to a lesser extent phosphates, the two primary nutrients that this invasive green algae needs to live in your koi pond. Both of these are naturally present in tap water and nitrates is a direct product produced by the koi fishs normal metabolic process.

Blanketweed loves warm weather, and so it will often flourish in your pond during the summer. The warmer the water is, the faster this green algae will be able to multiply.

Blanketweed Prevention In order to control the spread of this floating algae in our pond, we need to concentrate on cutting off one or more of its essential requirements.

Blanketweed depends on three things for its survival; sunlight, nutrients and a comfortable temperature. If one of these is cut off, this green algae will not be able to sustain its growth.

Practically we have to look at the first two options.

Cutting out Pond Sunlight

We can reduce the penetration of sunlight in our koi pond through a couple of methods. Firstly, it’s possible to combat this floating algae the traditional way, by shading the pond. Use a pergola or some other kind of screen to block the sun’s rays from the pond, as this will act to considerably reduce photosynthesis for the blanketweed.

The other way to reduce sunlight in your pond and therefore reduce floating algae is by adding vegetable dyes to your koi pond water. Adding these dyes will give a tint of color to your pond water, and will provide strong filtration of the sun’s rays. The only downside is that the dye will need to be replenished regularly as it will be slowly filtered out by your filtration system, but otherwise it has proven to be a very effective method of blanketweed control.

Reducing Nutrients that Green Algae Blanketweed Need

In order to remove the nutrients (Nitrates and phosphates) that the dreaded floating algae depends upon, you will need to use a special pond treatment that can be bought online or at a koi fish store. These treatments work by binding up the nitrates and phosphates that are present in your goldfish pond water.

They tend to be non permanent solutions.

Other Blanketweed Control Methods

There are a few other tactics we can employ in the fight against blanketweed green algae in our koi ponds.

One of the more popular ones (but least desirable) is to use algicides – chemicals that can kill algae by upsetting its biological processes. Alternatively, a more natural green algae killer is possibly barley straw. This traditional method is quite slow however and far from a universal solution. It is thought to be more eco-friendly.

The process is (supposedly) to place barley straw in your pond, and then, as it slowly degrades it will release hydrogen peroxide into your pond water, which is highly effective in reducing algae growth.

Rather than using real barley straw, it is possible to buy barley straw extract, which speeds up the process by a few weeks. One final method of control is through the use of an electric green algae controller. These supposedly work by upsetting the floating algae’s metabolism by altering the calcium ions in your pond water. Maybe black magic is a better explanation 


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