Green algae in a pond: properly sizing UV-C, filtration, and plants to eliminate them

Green algae are the main reason people contact us about a pond. In most cases, the owner has already “tried everything”: a product, a water change, sometimes even a UV lamp. And the water is still green. The problem is almost never a lack of equipment, but equipment that is incorrectly sized or installed in the wrong order. To eliminate green algae from a pond permanently, you need a UV-C clarifier with power suited to the actual volume, a pump and filter that circulate and retain what the UV has flocculated, and enough plants to deprive the algae of nutrients. Here is how to calculate each of these three pillars.

Two types of “green algae”, two solutions

Before sizing anything, take a close look at what is green.

• The water itself is green, like pea soup, and you cannot see the bottom at 20 cm: these are microscopic algae suspended in the water, or phytoplankton. This is where UV-C reigns supreme.
• The water is clear, but the walls, stones, and plants are covered with a green coating or green threads: these are attached or filamentous algae. UV has no effect on them, since they never pass through the unit. They are controlled by manual removal, plant competition, and nutrient reduction.

Often, both occur together: the green water is corrected with UV, then you notice that the filamentous algae are benefiting from the restored light. This is not a failure; it is the next step. To understand the causes in detail, our article “Why do I have green algae in my pond?” complements this article.

Step 0: determine the actual volume

All calculations start with the volume, and this is the value most often estimated incorrectly. A “10 m³” pond is often actually 6 or 7, because the length, width, and maximum depth were multiplied without taking shelves and slopes into account. Quick method: average length × average width × average depth, in meters, multiplied by 1000 to obtain liters. For an irregular shape, a coefficient of around 0.6 to 0.7 is often applied to the bounding rectangle. If you have a water meter, note its reading before and after filling: this is the most reliable measurement.

Pillar 1: UV-C power

The UV-C clarifier contains an ultraviolet lamp protected by a quartz tube. The water flowing around it is exposed to UV rays, which damage the cells of suspended algae. These clump together into larger particles that the filter then retains. The water clears in one to two weeks if the unit is suitable.

Each manufacturer indicates a maximum pond volume for each model, generally with two values: without fish and with fish, or ornamental pond and koi pond. Choose the more conservative value that corresponds to your pond, and allow an additional margin if the water is in full sun for a large part of the day.

Install it correctly

• Position: after the pump, before the filter. Algae clumped together by the UV must be retained by the filter's foam.
• Flow rate: observe the maximum flow rate indicated. Water that passes through too quickly does not receive enough UV. If your pump is very powerful, a bypass may be necessary.
• Operation: run continuously during the season, around 24 hours a day as long as the water tends to turn green.
• Maintenance: clean the quartz tube when it becomes cloudy, as limescale or organic deposits block the UV. Replace the lamp according to the duration specified by the manufacturer, often once per season: it continues to light up even though its UV output has decreased.

Pillar 2: the pump and filter

A UV-C unit without a filter does little to clear the water: clumped algae remain suspended or settle on the bottom, where they decompose and release nutrients. The mechanical filter traps them, while the biological section processes the fish's nitrogenous waste.

Beware of the word “actual”: a pump's stated flow rate is measured without any lift or hose. As soon as the water has to rise one or two meters to a gravity-fed filter or waterfall and travel through 10 or 15 meters of hose, the effective flow rate drops significantly. Check the manufacturer's flow rate/head curve.

For filtration, a combined model with integrated UV simplifies installation for typical-sized ponds. For larger or heavily stocked ponds, a separately sized filter with an external UV unit of suitable power offers more leeway. A phosphate-removing medium in the filter can speed up the return to balance if a test shows excess phosphates, which is common in older ponds or ponds supplied with mineral-rich tap water.

Pillar 3: plants, or how to starve algae

UV and the filter treat the symptom; plants treat the cause. They consume nitrates and phosphates, provide shade and limit water warming. In a well-planted pond, UV often becomes a spring supplement rather than a permanent necessity.

• Floating plants and plants with floating leaves (water lilies, as well as seasonal floating plants): ultimately aim to cover around one-third to one-half of the surface.
• Submerged oxygenating plants: several bunches per square metre of shallow-water area. They compete directly with algae for dissolved nutrients.
• Marginal and marsh plants (irises, rushes, water mint…): their roots in the water filter and absorb nutrients, while shading the edges.

Plant in a poor substrate (sand, gravel, baskets without garden soil): enriched potting soil would release exactly the nutrients you are trying to remove.

The order of action

1. Remove what feeds the algae: dead leaves, thick sludge, excess food. Reduce the fish's rations during the treatment phase.
2. Install or adjust the UV-C and filter, then let them run continuously.
3. Do not empty the pond: completely refilling it with tap water adds new nutrients, destroys the biological balance and triggers another green water phase a few weeks later. Prefer partial top-ups of around 10 to 20%, using rainwater if possible.
4. Plant as soon as the season allows.
5. Be patient: one to two weeks for green water, one season for complete balance.

If the pond is truly overrun, with algae attached everywhere, a thorough cleaning may be necessary before sizing the equipment: the method is described in our guide to cleaning a pond full of algae.

The most common sizing mistakes

• UV calculated based on the “estimated” volume instead of the actual volume, or without taking the fish into account.
• UV pump too powerful: the water passes through too quickly and the treatment is ineffective.
• Pump too weak for the pond: some of the water almost never passes through the UV, especially in areas far from the intake.
• Lamp never replaced: it still lights up, but no longer treats effectively enough.
• Chemical algaecide in addition to the UV: the sudden death of a large mass of algae consumes the water's oxygen. If you use a treatment, follow the dosage and aerate.

And what about barley straw? Its effect is debated and depends on many conditions; it can support a strategy but cannot replace properly sized equipment. For an approach that prioritizes biological solutions, also see our natural methods for eliminating algae from a pond.

Real-world example: an 8 m³ pond in full sun

Common case: a free-form pond advertised as 12 m³, around ten goldfish, facing due south, a small fountain pump, and a UV lamp bought "at random." The corrected bounding-rectangle calculation actually gives approximately 8 m³. The right setup would look like this: a UV unit rated for at least 8 m³ with fish, with some margin for sunlight; a pump delivering an actual flow rate of around 4,000 to 5,000 L/h, compatible with the UV unit's maximum flow rate; a biological filter designed for this volume with fish; and finally, water lilies and oxygenating plants to gradually cover one third of the surface. With this setup, green water generally disappears within a few weeks and only returns briefly in spring.

The spring green phase: normal, but brief

Even a well-equipped pond can turn green for a few days in spring. The water warms faster than the plants start growing again, light levels increase, and the filter bacteria are not yet fully active after winter. This is not a sign of incorrect sizing. Restart the UV early in the season with a new lamp, feed the fish sparingly while the water remains cool, and let the plants recover. If the green phase lasts more than a few weeks while the water remains above 15 °C, that is when you should review the UV power, flow rate, or fish load.

Also read

To understand the origin of the problem: Why do I have green algae in my pond?.

READ THE ARTICLE

Frequently asked questions

— What UV-C power do I need for my pond?

The one whose datasheet covers your pond's actual volume, using the “with fish” or “koi” rating if necessary, and going one step higher if the pond is in full sun.

— Does a UV lamp eliminate filamentous algae?

No. UV-C only acts on suspended algae that pass through the device, which cause green water. Attached algae are tackled by manual removal, plants and nutrient reduction.

— How long does it take to get clear water again after installing a UV?

Generally one to two weeks if the UV, pump and filter are correctly sized and operate continuously.

— Do you need to empty the pond to get rid of green algae?

No. A complete refill with tap water adds new nutrients and often triggers another green-water phase. Prefer partial top-ups.

In summary

• Distinguish green water (UV-C effective) from attached algae (UV ineffective).
• Calculate the actual volume, then choose the UV-C according to the manufacturer's datasheet, allowing a margin for sunlight and fish.
• Place the UV before the filter and respect its maximum flow rate.
• Run the volume through the filter in one to three hours depending on the fish load.
• Plant to eventually cover one-third to one-half of the surface: this is the sustainable solution.

Last updated: October 3, 2026.