Phosphates in the pond: why phosphorus feeds algae, and how to reduce it sustainably

You have a good pump, a proper filter, a new UV lamp, and yet filamentous algae return every spring. In the vast majority of cases, the culprit is invisible: it is dissolved in the water. Phosphorus is the element that most often determines how much algae a pond can produce. Understanding where it comes from means understanding why some ponds stay clear effortlessly while others never do.

Why phosphorus and not nitrogen?

To grow, algae need light, carbon, nitrogen, phosphorus, and a range of trace elements. Their growth is limited by the least available ingredient, just like a bricklayer who is missing one essential material. In most freshwater, that missing ingredient is phosphorus. Experiments conducted on entire lakes by Canadian limnologist David Schindler in the 1970s demonstrated this spectacularly: one half of a lake enriched with phosphorus turned green, while the other half, which received only nitrogen and carbon, remained clear.

A garden pond works in the same way, but on a much smaller scale and therefore much faster. Just a few hundredths of a milligram of phosphate per liter are enough to fuel a strong growth of filamentous algae. Conversely, a phosphorus-poor pond remains clear even in full sunlight.

Where does the phosphorus in your pond come from?

Sludge, the hidden reserve

This is the point most often overlooked. Phosphorus does not disappear: it accumulates. A large part binds to particles and settles on the bottom, where it appears harmless. But when oxygen becomes scarce near the bottom, particularly in summer during hot, stormy weather, the chemical bonds holding it are broken and it returns to the water. Limnologists call this internal loading. That is why a pond can remain green for years after its feeding has been corrected: it feeds on its own bottom.

Good aeration and regular sludge removal break this cycle. Specialized bacteria also help digest accumulated organic matter.

Measure before taking action

A phosphate test using drops or strips costs little and prevents blind treatment. Test the pond water, but also the water you use for top-ups: if it already contains phosphates, every refill will trigger algae growth again. As a guideline, aim for a value as low as possible; above 0.1 mg/l of phosphate, algae have ample opportunity to grow. Bear in mind that a low test result is not always reassuring: in a very green pond, the phosphorus is already stored in the algae themselves rather than dissolved in the water.

Six ways to lower phosphorus over the long term

1. Feed less, and better: an amount suited to the water temperature, eaten within a few minutes, and digestible food that produces less waste.
2. Prevent debris from entering: use a protective net in autumn and collect leaves before they sink.
3. Remove the sludge once or twice a year, using a pond vacuum or a net.
4. Rely on plants: they absorb phosphorus to grow. Prune them and remove the trimmings from the pond, otherwise the phosphorus will return to it.
5. Oxygenate the bottom to prevent release from the sludge.
6. Bind phosphorus with a phosphate remover when other measures are no longer enough or to get over a hump.

Mistakes that keep the problem going

• Use ordinary garden fertilizer for aquatic plants instead of fertilizer tablets pushed into the substrate.
• Planting in rich garden soil, which releases its nutrients into the water.
• Leaving removed algae or pruning waste on the bank, where rain carries it back into the water.
• Emptying and thoroughly cleaning the entire pond: useful bacteria are lost, and new water may itself introduce phosphates.
• Relying solely on a UV lamp: it acts on suspended algae, not filamentous algae, and does not remove any phosphorus.

Read also

To take action: How to easily reduce filamentous algae with a phosphate-removing solution?.

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Frequently asked questions

— What phosphate level is acceptable in a pond?

As low as possible. In practice, a phosphate level below 0.1 mg/l strongly limits algae; above that, they have enough to proliferate.

— Is a phosphate remover dangerous for fish?

Products designed for ponds can be used in the presence of fish when dosed according to the actual water volume. Always follow the instructions.

— Should the water be changed to reduce phosphates?

Only if the replacement water contains less than the pond water. Test it first: tap water with high phosphate levels makes the situation worse.

— Are plants enough to remove phosphorus?

They contribute significantly, provided there are enough of them and they are pruned regularly. In a pond heavily stocked with fish, they need support from the other measures.

In summary

• Phosphorus is the nutrient that most often limits algae growth in freshwater.
• It mainly comes from food, debris, soil, fertilizers and sometimes tap water.
• It accumulates in the sludge, which releases it when the bottom lacks oxygen.
• Feed less, remove the sludge, plant, oxygenate and bind: this is the combination that works.

Last updated: September 28, 2026.