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
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
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
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
Sunlight. Sunlight penetration can be reduced in a number of
Shading. Erecting shading on a pergola will reduce sunlight
straight away and reduce blanketweed photosynthesis. It can also
help against heron predation.
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
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!
pond algae blanket koi water filter growth nutrients sunlight
plants filaments balance installing UVc environment