Kill pond algae string algae

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.

Without doubt, nuisance algae tops the list of UK pond owners’ complaints. Green water needn’t be a problem for any pond owner who has re-circulating water in their pond since ultra violet units (adapted from the food processing industry) have been introduced to the pond and aquatics industry. Yet taking its place, we have seen the resurgence of blanketweed, rising through its ability to colonise a pond, often reaching plague proportions.

Requiring water, sunlight and nutrients to grow, blanketweed will even thrive in a bucket if given the necessary conditions. As should always be the case with responsible pond keepers who are guardians of their fish, we should identify and address the cause of blanketweed in our ponds and only then, having tried preventative methods, unlock the treatment chest and try to counter it with a pond additive.

The reason why we should make our own preliminary investigations into the cause of the blanketweed (concluding on either light or nutrients or both – as every pond’s environment will be different) is that there is a cost (not just financial) when using a counter measure or treatment against algae. As described earlier in this series, any compound that is added to a pond will affect the pond’s chemistry and depending on a number of factors, will have an effect on our koi.

Why are more ponds than ever plagued with blanketweed?

Blanketweed is by definition, a weed, that is, a plant that is growing out of place. If it grew in our garden (like a dandelion or nettle), then we could simply dig it up or hoe it over and plant more desirable plants in its place. Blanketweed’s success is attributable to several factors. Firstly unlike garden weeds, it does not have a root system. Blanketweed can be so widespread in a pond, yet we still quite literally cannot get to the root of the problem and pull it out in one piece. In fact, a single strand of blanketweed can soon become attached to a new surface.

Of the two factors that enhance blanketweed growth (sunlight and nutrients) nutrient levels in the water must account for the steady increase in blanketweed problems in the UK. After all, the sunlight will not have changed in its intensity or duration. I have been trying to research the typical levels of nitrates and phosphates in tapwater in my region 10 or even 20 years ago in an attempt to compare them with current levels being supplied from the tap.

My gut feeling (which at this stage is unproven) is that today’s tap water is higher in algae-promoting nitrates and phosphates than it was all those years ago. Another factor that could have lead to the increase in nutrients is the level at which we feed our fish compared to previous years.

Pond owners are demanding ever high specification diets, packed with nutrients and minerals, which will unavoidably lead to increased nitrates and phosphates in pond water. The outcome of these factors will lead to a pond’s increased susceptibility to blanketweed. Furthermore, the increase in the number of unplanted koi ponds in the UK, which offer blanketweed a complete monopoly on the sunlight and nutrients, will also have lead to its notoriety.

Blanketweed. A case of ‘if’ or ‘when’?

I remember overhearing an aquatic trader trying to sell his treatment for blanketweed to passers-by at a koi show. His sales patter went something like this: ‘Do you have blanketweed in your pond sir?’ and when offered the reply by his potential customer, ‘No, I don’t have any blanketweed in my pond’ the trader carried on, ‘Well when you do, come back and try our product’. I am sure most of us will agree with the trader’s sentiment. It is not a matter of if, but rather ‘when’ we get blanketweed. So when you do need to try a pond additive to treat blanketweed, what side effects might they have on your pond water as they work hard against the ensuing blanketweed?

Algicides. Chemicals that kill algae, by interacting with the plant at the cellular level.

When used at the correct and regulated concentrations, algicides will not cause any noticeable direct side effects on fish. When they are dissolved in pond water, algicides are absorbed by plants, retarding their growth as their cells are killed. The active chemical is broken down over several weeks by bacteria and particularly UV light, reducing its concentration steadily to levels which make it ineffective against blanketweed. The fact that it is broken down by bacteria over a matter of weeks should make it apparent to us that additional aeration during treatment should be applied to the pond. Furthermore, such additional aeration will prevent the DO levels dropping as a result of the microbial breakdown of dead and dying blanketweed. For this reason, mechanical filter chambers should be purged frequently as sediment collecting in a filter will have the same impact on DO as if it were lying on the pond bottom.

Additional aeration

During the last spell of hot weather, a number of koi keepers contacted the Nishikoi Infocentre experiencing problems with gasping fish having recently treated their pond. After initial enquiries, the ponds were well aerated with either venturis or diffused air. However, the addition of treatment to their ponds had put an extra burden on the DO level of their pond, requiring additional aeration over and above any existing aeration which may only just be coping with the stocking levels under normal conditions. Extra aeration means adding aeration, over and above the current levels.

Chemical phosphate or nitrate removers.

These treatments bind up and remove nutrients that are usually freely available to the blanketweed, fuelling its growth. These chemicals have very little effect on water quality and because they work by stealth, controlling and reducing algae growth (rather than killing it), additional aeration during its application is not as vital for the well being of the pond as when an algicide is used.

Microbial Treatments

A new generation of blanketweed controllers harnesses the action of nutrient-removing bacteria. Working to achieve the same result as chemical nutrient removers, bacterial cultures work in a completely different way. A bacterial or microbial method of removing nutrients from the pond is usually easily identified by the tiny amounts that are required to treat a pond. The bacteria are either ‘freeze-dried’ or suspended in solution in an inactivated state.

Upon immersing in water, the activated bacteria start to divide and multiply as they digest and gain sustenance from the nitrates and phosphates in the water. As these remedies are completely natural, adding very little in the way of chemicals to the water, they pose very little threat to fish and water quality. However, their performance can be improved by the provision of aeration, with the bacteria concerned thriving in well aerated water. As such microbial controls address the cause of algae problems, they too do not cause algae to die, just retarding its growth, reducing the risk of deoxygenation from dead and dying algae.

There is a wide range of pond treatments available to address blanketweed problems, each having their own key features and benefits. As every pond is different and each koi keeper’s experience will vary, there is not one single treatment that will be 100% effective all of the time. In addition, when selecting one of the many blanketweed treatments, we should also be well aware of any of the potential side effects and plan or intervene accordingly to reduce its effects on the life of our ponds and koi.

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