Pond UV Clarifier Sizing Chart
Pond UV clarifier sizing chart by gallons: about 10 watts of UV-C per 1,000 gallons for shaded ponds and up to 18 to 20 watts per 1,000 gallons in full sun, from 250 to 10,000 gallons.
Quick answer: Size a pond UV clarifier at about 10 watts of UV-C per 1,000 gallons for a shaded pond, and up to 18 to 20 watts per 1,000 gallons for a pond in full sun or stocked with koi. A 1,000-gallon shaded pond needs roughly a 10-watt unit; the same pond in full sun wants closer to 18 watts. Run it continuously and match it to your pump flow.
Pond UV clarifier sizing chart
| Pond volume (US gallons) | Shaded / partial sun | Full sun or koi pond (recommended) |
|---|---|---|
| 250 | 3 W | 5 W |
| 500 | 5 W | 9 W |
| 750 | 8 W | 14 W |
| 1,000 | 10 W | 18 W |
| 1,500 | 15 W | 27 W |
| 2,000 | 20 W | 36 W |
| 3,000 | 30 W | 54 W |
| 4,000 | 40 W | 72 W |
| 5,000 | 50 W | 90 W |
| 7,500 | 75 W | 135 W |
| 10,000 | 100 W | 180 W |
The shaded column applies the 10 watts per 1,000 gallon rule; the full sun column uses roughly 18 watts per 1,000 gallons. Because commercial UV units come in fixed sizes (9, 13, 18, 25, 36, 55 watts and up), always round up to the next available size rather than down. An oversized UV simply clears faster, while an undersized one can never quite win against a bloom. Not sure of your gallons? Start with our pond volume calculator, then run the exact numbers on the UV clarifier calculator.
What a UV clarifier actually does
A UV clarifier solves one specific problem: green water. Pea-soup green is caused by free-floating single-celled algae, and as that water passes the UV-C bulb the light damages the algae cells so they clump and drop out, where your filter catches them. That is the whole job. A UV does nothing for string algae or blanketweed stuck to rocks, because that algae never travels through the unit. It also will not fix the underlying excess nutrients feeding a bloom, so pair it with sensible stocking, feeding, and filtration.
Pond UV clarifiers by size
Pond Logic UltraUV Pond Clarifier, 13 Watt
$149.99 on Amazon
EPA-registered UV with a vortex chamber for ponds up to about 1,300 gallons.
uvcoolplunge 20W Pond UV Clarifier (100 to 1,500 gal)
$58.99 on Amazon
Submersible UV-C unit with a 17 ft cord for koi ponds and water gardens.
Coospider 32W UV Pond Clarifier (200 to 3,000 gal)
$79.99 on Amazon
Dual-tube UV-C clarifier for larger koi ponds in full sun.
Flow rate and placement matter as much as watts
Wattage is only half the equation. A UV clarifier needs the water to dwell long enough under the bulb to absorb a killing dose of UV-C. Push too much flow through and the algae sails past before the light does its work. Stay inside the manufacturer flow range for your unit, and plumb the UV after your mechanical and biological filtration so dirt does not shade the quartz sleeve. Many pressure filters bundle a UV inside, which keeps the build simple.
Run the UV continuously through the algae season, and replace the bulb every 12 months even if it still glows, because UV output fades long before the lamp dies. Wipe the quartz sleeve when you service the filter. Sizing your pump correctly is the partner step here: your pump should turn the whole pond over at least once per hour, so check it on the pond pump calculator.
When the water stays green
If a correctly sized UV has run for a week and the water is still green, work down this list: confirm the unit is actually sized for your real gallons and sun exposure, check the bulb age, slow the flow if it exceeds the rated range, and clean any debris shading the sleeve. Heavy koi stocking and overfeeding overload even a good UV with nutrients, so reduce feeding and improve filtration alongside. Sized and maintained right, a UV clarifier is the most reliable cure for green water there is.
Frequently Asked Questions
How many UV watts do I need per gallon?
A common rule of thumb is roughly 10 watts of UV-C per 1,000 gallons of pond water for a shaded or partly shaded pond. Ponds in full sun, or those stocked heavily with koi, benefit from closer to 18 to 20 watts per 1,000 gallons because they grow algae faster. Always round up to the next available bulb size rather than down.
Does a UV clarifier kill string algae?
No. A UV clarifier only clears green water caused by single-celled planktonic algae floating through the unit. It does nothing for string algae or blanketweed clinging to rocks and liner, because that algae never passes the bulb. For string algae you need manual removal, better nutrient control, barley, or a dedicated algaecide, not a bigger UV.
Why does a sunny pond need more UV watts?
Sunlight is the fuel for the planktonic algae that turns water pea-soup green. A pond in full sun grows that algae faster and in greater volume, so the same water needs more UV-C energy to knock it back. Shaded ponds get less algae growth, so they clear with less wattage. Sizing up for sun prevents a unit that can never quite catch up.
How does flow rate affect UV performance?
UV clarifiers only work if water moves slowly enough to get a real dose of light. Push too much flow through and the algae passes too fast to be killed. Match the unit to your pump within the manufacturer flow range, and remember that a UV is plumbed after mechanical filtration so dirt does not shade the bulb. Replace the bulb yearly even if it still glows.
Where does the UV clarifier go in the system?
Water should pass through your mechanical and biological filtration first, then through the UV clarifier, then back to the pond. Cleaning out debris before the UV keeps the quartz sleeve clear so the light reaches the algae. Many pressure filters include a built-in UV, which keeps the plumbing simple. Keep the UV bulb away from direct skin and eye exposure during service.
How long until the water clears?
With a correctly sized UV running continuously and a pump moving the full pond volume past the bulb, green water usually clears within three to seven days. Some units show results in as little as 48 hours. If water stays green after a week, the unit is likely undersized, the bulb is old, flow is too fast, or debris is shading the sleeve.
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