Why is my pond water green?
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The pond was clear in spring. Now you cannot see an inch into it. Is that normal, and what do the UK wildlife and pond bodies say to do about it? You can check the tap side of the picture at your own tap with a 20-in-1 test strip.
Short answer: Microscopic algae feeding on sunlight and nutrients. The Freshwater Habitats Trust says a new pond can stay green and cloudy for 6 months or more until enough zooplankton, such as water fleas, build up, and the RHS says changing the water may solve the immediate problem but makes things worse in the long run.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
Why is my pond water green?
Because tiny algae are growing in it. The Royal Horticultural Society (RHS) says single-celled algae suspended in the water turn it a "pea soup" colour, and that algae do best in warm, sunny ponds with little planting to shade the surface, and in water rich in nutrients. The Freshwater Habitats Trust says the root cause is usually too many nutrients, especially nitrate and phosphate. A 20-in-1 WaterKit test strip can show nitrate, pH and hardness in the pond and at your tap, but it cannot tell you why the pond has turned.
How long does green water last?
In a new pond it can be 6 months or more, and in an older pond it can persist. The Trust says new or recently dredged ponds often have temporary pea-soup blooms for a few months, because the algae's natural predators, zooplankton such as water fleas, take much longer to build up. It says you can do little to speed that up, and warns against adding sludge or water from another pond. Persistent green water or duckweed cover in an older pond is, in its words, of more concern, and it links that to nutrient pollution.
Why might my tap water make the pond greener?
The area figure is an average. Your tap water, which many people use to top up a pond, can carry nutrients that feed algae. The Freshwater Habitats Trust says tap water can have high levels of phosphorus and nitrogen, and that this depends on where the water comes from. Upland water is typically nutrient-poor, while water from intensively farmed areas such as South East England is generally higher. It points to your water company's website for what is in your supply.
The Trust also says chlorine and chloramines are harmful to freshwater wildlife. Leaving tap water in a bucket for 24 hours lets chlorine leave, but does nothing about chloramines (see what is chloramine). The RHS says to top up with rainwater whenever possible because nutrients in tap water encourage algae, and the Wildlife Trusts and RHS booklet Ponds for all advises topping up with rainwater if levels drop low in summer. Our water butt guide covers collecting it, and the tap water in a tank or pond guide covers the tap side. To compare your own tap, butt and pond, dip a strip in each and set the nitrate pads side by side, or enter each set of colours in our results checker.
“If you top up your pond from the tap, I would not assume it is neutral just because the area looks fine on paper. Tap water varies with where your supply comes from, so comparing your own tap, your butt and the pond side by side is a sensible way to see what you are really adding.”
How do I clear green pond water fast?
We found no fast fix without a catch in the sources we read. The RHS says changing the water may solve the immediate problem but makes things worse in the long run, and that no chemical treatments or weedkillers are approved for home gardeners to control pond algae and blanket weed.
| Option | What the UK sources say |
|---|---|
| Hand removal | The RHS says to scoop out blanket weed with a net, rake or cane. The Trust says it also removes the nutrients locked in the plants, but duckweed can double its area every 3 days in summer, so it must be repeated and the waste dumped well away from the pond. |
| Filter or UV clarifier | The RHS lists both. The Trust says they work when properly set up but are expensive and need upkeep, and that UV types also kill other tiny pond animals. |
| Barley straw | The RHS suggests about 50g per sq m of surface in a mesh bag, added in spring and removed when it turns black, about 6 months later. The Trust says it takes 2 to 4 weeks to work and is a quick fix, not a cure. |
| Pump or fountain | The Trust says a pump circulating about 50 percent of the water per hour can work against many algae. The RHS says aerating can help prevent blue-green algae but is unlikely to affect other types. |
| Pond dye | The Trust says dyes block some of the light algae need and fade over a few weeks, but also kill the larger submerged plants that absorb nutrients. |
How do I improve pond water quality?
By cutting what feeds the algae, which the Trust says is in the long term the best solution. The RHS suggests these steps.
- Keep soil out of the pond, and plant in low-nutrient aquatic compost.
- Stop feeding nearby lawns and borders, or use the minimum effective dose.
- Net the pond in autumn to keep leaves out.
- Top up with rainwater rather than tap water.
- Keep the pond free of fish if you can, because their faeces add nutrients. The Trust adds that fish stir up sediment and eat the zooplankton that would otherwise clear the algae.
- For a new pond, choose part shade, a depth of at least 60 to 75 cm if possible and about half the surface covered by aquatic plants.
The Trust also lists removing bottom sediment, which stores nutrients, and the Wildlife Trusts booklet suggests doing silt work in autumn when the pond is less active. We found no UK official nitrate or phosphate target for a wildlife pond in the sources we read. The Freshwater Habitats Trust suggests comparing your tap water's nutrient levels with those found in ponds never affected by pollution, in its factsheet 'How do I assess and manage pollution in ponds', which we have not read.
What can a test strip tell me about a green pond?
It indicates nitrate, pH, hardness and chlorine in the water you dip it in. It cannot show the cause of the algae. Dip one strip in a clean glass of pond water, wait 30 to 60 seconds and match the pads to the chart, or enter the colours in our free results checker. We have not validated the strips on pond water, so treat a pond reading as an indication. The last column is WaterKit's own reading of the chart.
| Pad | What the sources we read say | Our strip chart |
|---|---|---|
| Nitrate | The Trust names nitrate as a main nutrient behind green water. We found no target figure, so compare the pond with your tap or butt water. | Steps at 0, 10, 25, 50, 100, 250 and 500 mg/l |
| pH | We found no source linking pH to green water. | Steps from 6.2 to 8.4, so it cannot show above 8.4 |
| Hardness | We found no source linking hardness to green water. | Steps from 0 to 425, unit basis unconfirmed |
| Free and total chlorine | The Trust says chlorine and chloramines are harmful to freshwater wildlife, and that standing tap water for 24 hours lets chlorine leave but not chloramines. | Steps at 0, 0.5, 1, 3, 5, 10 and 20 |
Several things are outside what the kit can do.
- It has no phosphate pad, and the Trust names phosphate alongside nitrate, so a low nitrate reading does not rule out a nutrient problem.
- It has no ammonia pad. The nitrite pad starts at 1 mg/l, so a zero reading means none detected at that step, not none at all.
- We do not claim the rapid bacteria test for pond water.
See our home testing guide and how to keep pond water clear.
The area figure for your supply is an average, and the Freshwater Habitats Trust says nutrient levels in tap water depend on where the water comes from, so your own tap may differ. The pads that matter here are nitrate and chlorine. A nitrate step that reads higher in the pond than at your tap or butt would point to nutrients arriving from somewhere other than the tap, and a chlorine reading at the tap shows what a top-up adds. Best pack: 20 strips, enough to test your tap, your butt and the pond several times through the season, £5.99 (or £10.99 with a bacteria test for your drinking water), with free 2nd Class delivery on every order. See the kit
Your green pond questions
How to clear green pond water fast?
The sources we read offer no fast fix. The RHS says a water change makes things worse in the long run, and the Trust calls additives quick fixes, not cures.
Why is my pond water green?
Algae growing on sunlight and nutrients, says the RHS. The Trust names nitrate and phosphate as the main nutrients behind it.
How to improve pond water quality?
Cut the nutrients going in from soil, fertiliser, leaves, fish and tap water, say the RHS and the Trust, and use rainwater for top-ups.
Sources
- Royal Horticultural Society, Pond algae and blanket weed (undated page; causes, nutrients, rainwater, fish, barley straw, filters, dye, no approved chemical treatments, new pond depth and planting; read 29 October 2026).
- Freshwater Habitats Trust, FAQs: Problems with algae and duckweed (advice sheet dated September 2013, copy hosted by Avon Wildlife Trust; nitrate and phosphate, new ponds, filters, barley straw, pumps, dye, hand clearing, fish; read 29 October 2026).
- Freshwater Habitats Trust, Water Saving Week: work that butt! (26 March 2015; tap water nutrients, chlorine and chloramines, water company website; read 29 October 2026).
- The Wildlife Trusts and RHS, Ponds for all, big or small (Wild About Gardens booklet, 2019 copyright; rainwater top-ups, silt in autumn; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, 20 strips £5.99 or £10.99 with a bacteria test, free 2nd Class delivery on every order) and results checker. The strip-chart column and the list of limits are WaterKit's own reading of the colour chart steps.