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Dealing with Green Pond Water

By: Dr Gareth Evans - Updated: 29 Jul 2019 | comments*Discuss
Algae Sunlight Decay Nutrients Nitrate

There are few things more disappointing for the water gardener than to see the water of your lovingly built pond turn soupy green. In the past, dealing with the curse of green water could often turn into a major headache, but today, combining some of the old, time-honoured remedies with a spot of modern technology can usually be relied on to sort the problem out, once and for all.

Green water is caused by the growth of large numbers of single-celled algae, which live suspended in the water and is fuelled by light and excess nutrients in the water – typically making its appearance as the days begin to get sunnier in the spring.

Although it is most commonly encountered in recently constructed ponds, where the water chemistry and planting has not yet settled down, older, more established water features are not immune.

The principal culprit is nitrate, a nutrient which forms naturally as left-over fish food, fish faeces and dead plant material decays. Nitrate test kits are widely available and very simple to use, but often simply clearing out fallen leaves, cutting and discarding dead water-plant foliage and avoiding over-feeding your fish can make a big difference.

Natural Control Methods

A variety of methods can help to deal with green water. Natural methods include avoiding fertilising the pond itself – nor the area around it – removing decaying matter, not over-stocking with fish and sensible planting.

Surface plants compete with algae for light and so having as much as a half or two-thirds of your pond covered with vegetation can make a big difference, while trees or bushes can also be used to give shade, further helping to reduce the levels of sunlight reaching the water. However, these approaches may not always be enough in themselves, as green water may still happen in early spring, before the protective plants have developed new foliage.

For another natural additive, barley straw could fit the bill. Administered either as pouches of straw, or doses of straw extract, it is an approach which some have hailed as a miracle cure – though not all pond-keepers are unanimous in this opinion. It takes a month or two to show any effect, but the natural enzymes are said to have remarkable abilities to inhibit algal growth – making it effective against blanket weed also.

Chemical Treatments

Chemical treatments are also available to deal with the problem. Although they can be expensive and do not provide lasting protection, they can be very effective, particularly for newly constructed ponds. They must be chosen carefully if the pond has already been planted or stocked with fish and if you do decide to go down this route, then reading the label carefully and using the product in accordance with the instruction is obviously critical.

Perhaps the single greatest advance in recent years in the battle against green water has been the development of reliable and affordable ultra-violet (UV) clarifiers. Aside of its effectiveness in destroying the algae which cause the problem, this approach has a number of other advantages. It has no effect on water quality and because it takes place away from the pond itself, it poses no threat to the plants or fish. Easy to install and cheap to run, a good quality UV unit combined with a biological filter system and an appropriate planting regime offers the best possible means of dealing with the green water scourge – though it is important to remember to change the UV bulbs every year. A number of manufacturers produce UV units to suit a range of pond sizes, widely available from garden centres and other retail outlets.

Left unchecked, green water algae can turn the most beautiful of water features into a pitiful sight in a very short time, but the good news is that with a little careful thought and a bit of practical effort, it should be fairly straightforward to keep your pond crystal clear.

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Hi my pond water is completely clear but the water feature that runs into the pond through a filter/UV is thick with algae.I clear it almost daily.Can anybody offer any reasons for this.
Me - 29-Jul-19 @ 9:07 AM
My small pond has turned very green and fish dying Have cleaned pump and filter and filled with tap water , some fish appear to have survived.I do have quite a lot of sludge on the bottom of pond, I don’t think I have over fed them not many plants but loads of green weed forming very quickly. Is there something I can put in the water like blanket answer which has been successful in the past but 1st time water has turned green, the pond is in a warm sunny spot. Any advise I would be most grateful for Thankyou. Aud
Aud - 6-May-19 @ 6:12 PM
Hi, looking, for assistance. I've had a pond established for 4 years, it's just under 2000ltrs (7ft x 3ft x 3ft) and have 2 x hozelock easy clear 6000 running on it each with a 9w uv (18w combined). I've never had clear water, I've used barley straw, have elodea in it, it's partially shaded, I've now added a Oase biotec 10 on top of the 2 x easyclear 6000's and nothing works. I'm close to breaking it down and giving up. Help!
Si78 - 26-Jun-18 @ 7:35 PM
Hi I wonder if you can help, I have a pond 11feet in dia and about 3feet, I have fish in pond plus newts, my pond is turning green I have a ultra violet cleaningbox plus a water pump with a fountain on it. How do I stop my pond turning green (like pea soup). What products should I use please.
Ivor - 24-Jun-18 @ 9:01 PM
I have a home made pond using pond liner. What I would like to know is I have clear water coming out the filter but pond is green. What can I do to make the pond clear.
D williams - 18-Jun-18 @ 3:34 PM
Hi I have a small pond in my garden with crystal clear water,with just a normal pump with no filter, it is part shaded with some conifers, but the main factor I think for the clear water is my oxygenating plants and lilies that I have to remove at times if I want to see my fish. Now we have a near friend who has a larger un-shaded pond in her garden who,s water is very green and murky, very dissapointing as she has some lovely large koi that she is unable to see, I think the problem here is the lack of plants to cover the water to reduce the sunlight. she has an up to date filter system and has tried barley straw with no difference to the water colour, a local park has a large pond in direct sunlight with crystal clear water, with a central tall antiguelooking fountain, but the surface in the middle is covered thickly in the long oxygenating plants. So I intend to advise her to introduce a large amount of these plants, and reduce them over a period of time as the water hopefully gets more clearer. I will hopefully keep posting to let you know how we get and see if the algae reduces in her pond. Yokel John. .
yokel john - 12-Jun-18 @ 7:45 AM
Ah, green water.After having 3 new ponds this year I did some research as I like 'natural'.After making my veggie filters, and having one bio filter running until the veg filters kick in, I added Daphnia.Daphnia are the natural way to clear the algae as they feed and multiply on it.No green water, free fish food. Not everything is complicated if you worth with naurure :)
JJ - 9-Sep-17 @ 11:53 AM
I've inherited a small garden pond which has plenty of frog activity. It's recently become a bit overrun with algae so I have bought some barley straw after reading up but I dint have any running water or a pump in the pond.Can I still use it?
Ruth - 22-May-17 @ 11:39 AM
Make sure the water is well oxygenated - it needs plenty of oxygenating plants such as elodea. Then if or when it goes green use natural products such as Nishkoi blanket weed treatment.
nick name - 19-Jul-16 @ 4:10 PM
It is absolutely differing waste which can sink into the pond. With a couple of materials, Pondpro2000 pond liner could likewise make it less difficult to scour the upside.
Pondrepair - 30-Mar-16 @ 12:57 PM
Griselda - Your Question:
We have a small plastic pond on the allotment. It's sited in a fairly shady place but still gets some sun, more in spring time before leaves appear on the trees. We want to encourage frogs - last year they laid lots of spawn but it all died or disappeared in the green peasoup algae bloom. We want to prevent that happening again. The frogspawn is there. We have no electricity to run any filters or units. Could try a solar powered fountain and will plant some water lilies and try to find some barley straw. Also have put a pallet across the whole pond but any other suggestions would be very welcome.

Our Response:
There has been a very similar question in this section which you may find answers your question.
PondExpert - 24-Mar-16 @ 12:24 PM
We have a small plastic pond on the allotment. It's sited in a fairly shady place but still gets some sun, more in spring time before leaves appear on the trees. We want to encourage frogs - last year they laid lots of spawn but it all died or disappeared in the green peasoup algae bloom. We want to prevent that happening again.... The frogspawn is there. We have no electricity to run any filters or units.Could try a solar powered fountain and will plant some water lilies and try to find some barley straw. Also have put a pallet across the whole pond but any other suggestions would be very welcome.
Griselda - 22-Mar-16 @ 2:48 PM
I have two large and five medium goldfish in my pond which has gone from crystal clear to pea soup my question is how on earth do they see anything and does this p*** them off.
david - 25-Jul-15 @ 6:44 PM
please help my pond which ive had for just over four yrs has this yr become dirty with sediment in the water and algie wont grow causing sediment to be on side and ledges which everytime a fish goes on or near the sides they disturb the sediment causing the water to be dirty any tips tips to cure this thanks
kitch - 12-Jul-15 @ 4:26 PM
We had the same problem brand new pond set up within 3 days green pea soup. Purchased a UV system and within a week total clear beautiful water. Make sure to buy the correct size UV system truly a great purchase
moan - 9-May-15 @ 5:34 AM
My pond was clear, but I changed my pond filter. My old one had a very fine sponge like filter that worked under pressure and needed cleaning every week.But it worked.My new bigger unit has filters with much larger pores and it does not do the job...
Dickie - 26-Apr-15 @ 10:05 AM
Hi I put some food dye in the water and that worked for me but the water has a tint of blue
dicky - 12-Jun-13 @ 6:29 PM
I have tried all of the above ,including 4 treatments by well known makers of pond treatments,all to no avail.Is their anything that will work,other than emptying the pond and restarting?It has cost me a fortune in uv lights,chemicals etc,all to no avail.
acer - 11-Jul-12 @ 9:29 AM
Just moved in house with a pond, water was clear, no pump, filter etc and just 1 fish so brought another fish to keep company and some oxygenating plants to hopefully keep water oxygenated, within a week the water is cloudy green and want some natural solution to sorting this untill I am able to get pump etc
Gt - 9-Jul-12 @ 8:34 PM
I'd recommend the UV filter. It worked on my pond where nothing else had. I'd tried chemical treatment, albeit reluctantly, and with no success (I only did it because there were no fish in the pond). However, UV cleared the algae away pretty quickly and after that the plants were still healthy, so there was no detrimental effect whatsoever. If you've been having problems others treatments won't help, try that.
Eliza - 25-Jun-12 @ 1:53 PM
I like a lot of other pond keeper have done just about everything we can to get rid of green water, however I only have green water in my koi pond the other 7 pond's with various types of goldfish are all clear can you please tell me why
rockapeandproud - 5-Dec-11 @ 1:50 PM
Hi, I am emailing on behalf of my dad. We have a pond in our garden and we've been having trouble with the colour of the water for a while now. We have tried an endless list of things to clear it up. We've purchased extra UV lights and filters, we've tried some straw bundles that are meant to attract the algae to it, we've tried reducing the amount of water and cleaning what's in the pond then filling back up but we have seen very little or no improvement from any of these. I know that the change will not happen overnight but we have tried this for a number of years now. Do you have any advice or know where we can get some advice? Thanks very much
Laura - 3-Jun-11 @ 3:04 PM
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