What Size UV Clarifier for a 15,000-Gallon Pond?
A 15,000-gallon pond needs roughly a 220-watt UV clarifier to clear and prevent green water, based on about 10 watts per 1,000 gallons, more in full sun.
Quick answer
A 15,000-gallon pond starts at about 150 watts of UV using the 10 watts per 1,000 gallons rule. In partial sun, round up to a 220-watt clarifier; in full sun step up to 275 watts. Keep the flow through the UV near 10,000 GPH so the light has time to work.
UV wattage for a 15,000-gallon pond by sun exposure
| Sun exposure | UV clarifier size |
|---|---|
| Mostly shaded | 165W |
| Partial sun | 220W |
| Full sun | 275W |
| Target flow through the UV | ~10,000 GPH |
Green water in a 15,000-gallon pond is billions of free-floating, single-celled algae, and a UV clarifier is the simplest reliable cure. Sizing starts from about 10 watts of UV per 1,000 gallons, so this pond starts near 150 watts, then rounds up to a real bulb size and increases with sun exposure.
Size by Gallons, Then Adjust for Sun
Sunlight is the fuel for a green-water bloom, so a pond in full sun blooms harder than one in shade. Starting from 150 watts, a mostly shaded 15,000-gallon pond often clears with a 165-watt unit, a partial-sun pond wants about 220 watts, and a full-sun pond is better at 275 watts. Wattage is rounded up to common bulb sizes of 9, 13, 18, 25, 40, or 55 watts, and a slightly oversized UV simply clears faster.
Match the Flow, and Remember What UV Cannot Do
Wattage is only half the job. UV light needs contact time, so aim to turn the pond over through the unit once every 1 to 2 hours, roughly 10,000 GPH for a 15,000-gallon pond. Push water through faster and the algae survives the trip, so keep your flow at or below the UV unit's maximum rating. One caution: a UV only treats green water. It does nothing for string algae clinging to rocks and waterfalls, which needs manual removal and nutrient control instead.
UV clarifiers near 220W for a 15,000-gallon pond
Sized to clear green water at this volume.
Pressure filter with built-in UV
Filter and UV in one tidy unit.
Full-sun UV clarifier (~275 watt)
Step up if your pond bakes in full sun.
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Frequently Asked Questions
What wattage UV clarifier for a 15,000-gallon pond?
Using about 10 watts per 1,000 gallons, a 15,000-gallon pond starts near 150 watts. Round up to a 220-watt unit for partial sun, or a 275-watt unit if the pond sits in full sun, since more sun fuels faster green-water blooms.
Why does a full-sun 15,000-gallon pond need 275 watts?
Sunlight drives the algae bloom, so a pond in full sun blooms faster and harder. Bumping a 15,000-gallon pond from the 165-watt shade size up to about 275 watts gives the UV enough output to keep ahead of the bloom.
How fast should water flow through the UV on a 15,000-gallon pond?
Aim to turn the pond over through the UV once every 1 to 2 hours, roughly 10,000 GPH for 15,000 gallons. Too fast and the light cannot kill the algae, so keep flow at or below the unit's maximum rating.
Will a UV clarifier clear string algae in a 15,000-gallon pond?
No. A UV only treats green water, the suspended algae that drifts past the bulb. String algae stays anchored to rocks and waterfalls and never passes through, so it needs manual removal, beneficial bacteria, and nutrient control instead.
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