Blanket weed problems in my pond

Blanketweed or string algae is a big concern for many pond keepers. Green water is another unsightly algae problem, making a pond resemble pea soup. Blanketweed is another type of algae that forms an unsightly and smothering ‘blanket’ across the pond surface and throughout the pond. Blanketweed floats because of the oxygen that is trapped within it buoys it up. Blanketweed can smother a pond in days in perfect summer conditions, causing pumps to block up and submerged plants to be choked. Blanketweed thrives in clear water ponds, that are typically fitted with a UV clarifier. Blanketweed is best controlled by addressing the cause – which are the combination of nutrients (nitrates and phosphates), warm pond water and bright sunlight.

Dear Ben, I have an annoying problem with my pond and wonder if you can help. I have asked five different koi retailers and they have all come up with different solutions, none of which have helped. I have a lined pond approx 2,200 gallons. I have a Lotus 12,000 pump going to a TMC 25W UVC, then a Yamitsu 60W UVC, both with clean sleeves and new bulbs. This then goes into a large filter box with sponges and flocor, then returns to the pond on a waterfall. All hose is 1.25in. I have an air pump with two air stones and six large koi.

The problem I have is that there is a thick layer of algae all over the top of the water, although the water underneath the algae is clear and it is possible to see the bottom. It is only like this during the day and perfect at night. Would reducing the flow of the pump make any difference? Please Help.

Nothing is more frustrating to a pond keeper than nuisance algae, and from your description, it sounds as though your current algae problem is proving to be just that – a real nuisance.

You have confronted the algae problem which is usually the first to threaten all pond keepers which is green water, by installing two UVcs. These will work in conjunction with each other (and your filter) to flocculate and remove suspended algae – producing the crystal clear water that makes it ‘possible to see the bottom’.

However, because of the way the ecosystem of an artificial koi pond is by its very nature invariably unbalanced, nature’s own pressures try to balance it out again, and in doing so, sometimes causes other desirable side effects to occur. In your own case (and the majority of other ponds filtered with a UVc), the most common side effect is the proliferation of other nuisance algae – such as filamentous types (blanket weed).

So what’s really going on in your pond and why is your latest algae problem taking the form of a floating layer of algae in the day only? And having identified the possible causes, what are your potential solutions?

Why is algae still afflicting your pond?

A clear, filtered recirculating koi pond that is artificially stocked with koi relies on artificial inputs of food and a means of removing the excessive levels of the resultant waste (usually a filter). In comparison, a natural mud pond or lake embodies a natural balanced ecosystem that provides koi with an unrivalled, supportive environment that handles waste produced by koi and other pond organisms, reprocessing and transforming them into beneficial products.

Even though both types of pond set out to achieve the same result, an artificial koi pond does not achieve the rich and balanced environment that koi experience when grown in a natural mud pond. One of the starkest and most fundamental differences between the natural and manmade systems is due to the role that plants play in creating and maintaining a balanced pond environment.

In your filtered koi pond, you are striving to provide your koi with a plant-free existence, where your objective is to produce a pond of clean lines and low maintenance, where the only attraction in the pond is the koi themselves. But by taking this approach, you are denying your koi what mother nature had intended – the balance that can only be achieved through plants. Plants are to be found at the start of every food chain, performing a plethora of roles, from purifying water to acting as a food source. A natural pond benefits from plant life, and the environment in a filtered koi pond is compromised by living such a plant-free existence – we see this when algae of all types tries to rush in and fill the plant void.

Blanketweed is trying to fill the gap in your own koi pond. This void would usually be filled by aquatic plants in a natural pond. As koi keepers, we try to do our best to resist the growth of nuisance algae as strongly as possible, but the battle is relentless and we are usually happy to concede and settle for an uneasy truce.

What is Blanketweed?

Blanketweed is a collective term given to a number of very similar algae that both look and behave identically. The most common genera are Cladophora and Spirogyra.

The 3 key factors that stimulate its growth in your own pond will be:


Ponds in full sunlight are most likely to be affected. Light is required to fuel the process of photosynthesis, which allows these algae to produce organic molecules for new tissue growth. Growth is also particularly rampant in shallow areas where the water is exposed to intense sunlight, and your clear UV-treated water provides blanketweed with unhindered growing conditions.


Algae will readily absorb nitrates and phosphates to satisfy their need for nitrogen and phosphorous as they grow. These are readily available in tap water or indirectly through fish metabolism. Wherever nutrients abound, so will this opportunistic algae, being the first to capitalise on ideal growth conditions. Because your koi pond is an unnaturally stocked and relatively over-fed environment – nutrients will abound.


A high temperature will accelerate algae growth considerably and blanketweed growth will be greatest in the shallower areas such as cascades and waterfalls and along the pond perimeter.

Preventing Blanketweed from taking over

Some pond owners are fortunate in that blanketweed does not appear to proliferate in their ponds (and they cannot explain why). However, because of the factors listed above, the vast majority of us will find that our ponds are very hospitable to blanketweed, forcing us to look for ways of preventing it from causing problems.

The answer to controlling blanketweed does not lie in adjusting flow rates (as you mention in your letter) but rather lies in reducing one of their 3 key requirements; sunlight, nutrients, and warmth. As we want our ponds to be as warm as possible (to stimulate koi health and growth), we must look at reducing sunlight and dissolved nutrients.

Sunlight. Sunlight penetration can be reduced in a number of ways.

Shading. Erecting shading on a pergola will reduce sunlight straight away and reduce blanketweed photosynthesis. It can also help against heron predation.

Adding dyes.

Several blanketweed and algae controls work by adding dark vegetable dyes to the pond, filtering out the sun’s rays. This will give the water a tint, and will need to be topped up when the natural dyes are broken down by the filter.

Compelling natural evidence that shading works is evident when a pond suffers from green water. The microscopic single celled algae that turn a pond into a ‘pea soup’ out compete and shade blanketweed out of its valuable light. Blanketweed and green water have a mutually exclusive relationship, where ponds tend to suffer from either one or the other. Unfortunately, one of the side effects of installing a UVc (which is a guaranteed method of clearing green water), is that blanketweed will proliferate unhindered in the crystal clear, nutrient rich pond water. This is what you are currently battling against.


Several pond treatments are available that control blanketweed growth by locking up or removing the vital nutrients from the pond water, starving the growth of blanketweed. Upon adding to the pond, they will bind up nitrates and phosphates. Other additives will act indirectly, but achieve the same ends using micro-organisms rather than chemicals.

Why do you only get the problem during the day?

I suggest that the algae on the surface is in fact tiny broken filaments of blanketweed rising to the surface. In the daylight hours, plants photosynthesise to produce oxygen as a by-product which dissolves into the pond water. You can often see the evidence of the mass of fine bubbles that are released during photosynthesis by observing rafts of blanketweed rising to the pond surface in the day (buoyed up by oxygen bubbles that become trapped within the filaments), sinking to the bottom again at night. Rather than seeing large rafts of blanketweed, you are seeing the same phenomenon on a smaller scale with smaller broken filaments. The reduced size of filament is possibly as a desirable side effect of an existing blanketweed treatment that you have treated your pond with.

Why not try a skimmer?

A very effective mechanical means of removing this floating scum of algae would be to install a surface skimmer in the side of your pond. You will have seen these units in the side of swimming pool walls where they collect and intercept any debris that is attached to the pond’s surface. In your situation, a skimmer would physically remove the annoying algae as it floats to the surface of your pond in the daylight, helping to preserve the aesthetics of your pond. They can be retrofitted (you may have to drop the water level while you install it) and one of your 5 koi retailers will be able to advise you on their installation in your own particular pond.

In conclusion, I would be inclined to be grateful that compared to some major blanketweed problems that other pond keepers can experience, your algae problem of a daily raft of fragmented blanketweed is easily manageable. This can be controlled in several ways and should no longer impede your views through to the bottom of your crystal clear pond. Best of luck!

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