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Why Is My Pond Green and Frothy?

By: Dr Gareth Evans - Updated: 15 Jun 2020 | comments*Discuss
Green Water Algae Froth Protein Organics


Our pond has gone very green in the last month from being brand new clear water last August when we first built it. Consequently I noticed that the UV bulb had broken so I replaced it last week and cleaned the filter and pump.

Prior to cleaning the equipment and replacing the UV bulb, the pond had begun to produce lots of froth and bubbles. They begin around where the outlet pipe from the filter runs back into the pond. As you can see from the photo they are fairly large and they are not dispersing, creating a covering all over the pond. The bubbling seems to have got worse since changing the bulb and cleaning the equipment. Any ideas?

(T.Clarke, 20 March 2009)


It’s always a bit disheartening when a crystal clear pond seems to go wrong, but don’t despair – I don’t think you’ve got too much to worry about!

The Dreaded “Green”

Your pond’s still fairly young and you made it at the back-end of the summer last year, so I suspect it probably hasn’t fully matured and settled down yet. This can take a surprising amount of time, depending on pond size, depth and the weather – amongst other things.

You’ve added a biofilter and a UV lamp to your pond, so it sounds like you know all you need to know about the reasons for green water and how to cure it; I think you’ll just have to let it do its job over the next few weeks and hopefully, now the lamp is fully functional, you should find the problem clears up.

Check how much of your pond surface is covered by plants – too much light reaching the water at this time of the year can be a particular problem, since algae grow so very much quicker than other plants. If plant cover is too little – around a half, or even as much as two-thirds being about right – then you have the perfect excuse to enjoy yourself picking some new varieties at your local garden centre!

Froth Problems

By the sound of it, the froth is probably linked to cleaning out the filter – as well as the water not yet being fully matured. When you cleaned the filter elements, you probably (and unavoidably) stirred up all manner of sediment and attached biofilm material and I’m guessing that this is the at the bottom of the problem with the froth.

Protein-containing solutions froth easily and since you’re constantly agitating the water with the return from the filter, I suspect that this is the reason for the persistent froth you’re seeing at that point. The good news is that it’s actually not that uncommon and if this is the case, it should sort itself out as the filter bacteria re-establish themselves and begin to do a thorough clean of the water. If it seems to be going on too long, or if you’re not convinced, try testing the water – I’d put money on finding that the dissolved organic content is too high.

If there are any trees or other sources of organic material around your pond it might also be a good idea to take suitable measures to ensure that leaves, grass clippings and so on don’t make their way into the water, or this problem could recur – especially if the pond is a relatively small one.

Sorry you’ve got a problem, but if you keep running the filter/UV combination and can be patient, I suspect it will clear up on its own in good time. Enjoy your pond!

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We have had a wildlife pond made by my husband 3 weeks ago. It is filled with rainwater and measures 3 x 3 metres and is circular. We have 1 small and 1larger water lily that dont cover that much of the surface as yet. And for oxegenating plants. However it is already turning all green and slimy looking on top. Will this algae looking stuff kill the plants. What in your opinion can And should I do. Would be so grateful for your help and advice Many thanks
Goats rock - 15-Jun-20 @ 6:26 PM
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Cathy - 13-Jul-17 @ 9:25 AM
We have a new 100 gallon deck pond which has been running for about 3 weeks.I have 1 arum lily in it and a couple of growing water lilies - no fish. Already the water has turned green and there are white bubbles on the surface.We do have bricks in the pond to raise the level of the planters.From doing some research, it looks like a UV light could help return the water clear (chemicals did not help), and the pond is in full light.Does anyone know if all UV lights are mains operated?Reason I ask is that all the wiring for the pump has already been buried and landscaped over - I don't gfancy the idea of digging it all up again.....Can anyone help? Many thanks
shaken45 - 26-May-17 @ 4:11 PM
I have kept a Koi pond for 11 years with very few problems until now. Over the last two years the water has been green (except in the winter). The pond is 1400 galls with an Easy Pod fed by an Aquamax 8000. In desperation I have added a further pump. A pressure filter 12000lts fed by an Aquamax 5000 via a surface skimmer. Still green water. I have now added a dose of Answer Green away (about three days ago but no improvement as yet. My Koi are quite large (around 16) but seem quite happy. I have also carried out a partial water change. Oh! I also have a 30 watt UV with new tube and glass sleeve on the 8000 and another 11watt UV on the 5000.
Nigel Farris - 19-Jul-16 @ 8:22 PM
hi sorry been away working for a few days, the sediment is green and has now got quite thick growth on the sides of the pond also quite stringy. the pond is not yet a year old.
bigwig - 21-Apr-15 @ 9:00 PM
@bigwig. What colour is the sediment?
PondExpert - 16-Apr-15 @ 10:06 AM
can someone tell me why the plants in my pond seem to be covered in what looks like sediment that floats when disturbed. the water is pretty clear and I two uv filters working
bigwig - 14-Apr-15 @ 9:52 PM
I also hae froth on my pond sincethe weekend. I have treated it with green Algie remover without success.My pond is 4 M x 2 M and average depth 0.5 M.I have had it 15 years and the fish population has thrived from about 10 to over a hundred. They cannot stop breeding. Due to pump failure whilst on holiday I lost 12 very large goldfish [almost 3/4 of a pound.I have replaced the pump and clean the filter every 3 days but cannot get back to clear water. All the remaining goldfish seem very active. What can I do to get a froth free clear pond? Bill
Bill - 2-Jul-14 @ 9:49 PM
Hi, that was a very helpful about the green water, but I have green in my pond but it isall around the pond and base; that stringy stuff: and froth!We have just had it made a little bigger, so that we could keep two koi, twenty frogs and three new fish and twenty gold fish.?( we won`t discuss the tadpoles in another small plastic pond,about 200 of them, ) But the froth and green weed getting very bad and I did not want to start the chemical way of things, Can you suggest something other than spending all day getting it out winding it round a stick! ??????Oh for the joys of a pond????????????? Julie
Julie - 28-May-13 @ 9:49 AM
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