Quick answer: Size a koi pond UV clarifier by dose, not by wattage. The rule of thumb is roughly 17–18 W of UV power per m³/h of flow at typical clarifier efficiency, which for a 20 m³ pond with one turnover per hour means a 350 W unit, and for 40 m³, two 350 W units or one 700 W. Undersizing by 30% wastes money, because the lamp then delivers a dose too low to knock down green water and you end up running it 24/7 with nothing to show.
The Formula Behind the Wattage
A UV clarifier works by exposing free-floating algae to ultraviolet light long enough to damage their DNA. The meaningful number is not lamp wattage but UV dose — the amount of energy delivered per unit area of water, measured in mJ/cm².
The simplified industry formula is:
Dose (mJ/cm²) = 3.6 × UV power (W) × efficiency factor ÷ flow rate (m³/h)
The efficiency factor sits between 0.3 and 0.5, because quartz sleeves, water clarity, lamp ageing, and reactor geometry all absorb part of the output. A clean new lamp on clear water might hit 0.5; a sleeve that has not been wiped in two months drops toward 0.3.
| Target | Dose needed | Typical use |
|---|---|---|
| Green-water control (clarifying) | 20–30 mJ/cm² | The normal koi pond target |
| Bacteria and pathogen control | 30–50 mJ/cm² | High-stock ponds, quarantine loops |
| Clear water only, no algae pressure | 15–20 mJ/cm² | Lightly stocked ornamental ponds |
USEPA's Ultraviolet Disinfection Guidance Manual uses 30–40 mJ/cm² for disinfection duty; the clarifying target for commercial koi ponds is the 20–30 mJ/cm² band.
Sizing Table: Pond Volume to UV Power
Using the formula with a 0.4 efficiency factor, a 1× turnover per hour flow, and a 25 mJ/cm² target, the table below gives planning numbers. These are typical commercial values; actual sizing depends on fish load, feeding rate, and shading.
| Pond volume (m³) | Flow through UV (m³/h) | Calculated UV power | Recommended unit |
|---|---|---|---|
| 5 | 5 | ~87 W | 90–110 W |
| 10 | 10 | ~174 W | 180 W |
| 20 | 20 | ~347 W | 350 W |
| 40 | 40 | ~695 W | 2 × 350 W or 700 W |
Two notes on flow. First, the clarifier should see the full turnover, not a bypass trickle; a bypass loop at 20% flow delivers 20% of the dose budget. Second, running the pump at 2× turnover halves the UV contact time, so double flow means double wattage at the same target.
Where Most Installations Lose Dose
Buyers usually size the lamp correctly and then lose half the dose to three predictable failures:
- Quartz sleeve fouling. A dirty sleeve can cut UV transmission by 40–60% within a few weeks in a stocked pond. A wiper-equipped chamber, or a sleeve the maintenance crew actually wipes on a schedule, is the difference between 25 mJ/cm² and 12.
- Lamp ageing. UV output falls as a lamp ages; the common replacement interval is 8,000–9,000 hours, or roughly one season of 24/7 operation. Running a 14-month-old lamp is running a lamp at maybe 60% of rated output.
- Water clarity itself. Suspended particles shield algae from UV. A clarifier works best after mechanical filtration, not before it. Placing the UV upstream of a dirty drum or bead filter guarantees it is dosing cloudy water.
A Worked Example
A 20 m³ resort pond fought green water for a season (hypothetical, for budgeting). The first system was a 120 W clarifier on a bypass loop at 4 m³/h. Dose at 0.4 efficiency: 3.6 × 120 × 0.4 ÷ 4 = 43 mJ/cm² on paper — but the bypass carried only a fifth of the pond, the sleeve was cleaned twice a season, and the pond stayed green. Replacing it with a 350 W unit on the full 20 m³/h return, with a monthly sleeve wipe, delivered 25 mJ/cm² to the whole pond volume, and water clarity stabilised within 3 weeks. The electricity cost of the bigger lamp was about $12 a month — against $80 a month in flocculants and extra water changes the resort had been buying.
What to Ask Before You Buy
- What flow is the chamber rated for at the dose I need? Ask for the dose at your flow in mJ/cm², not just the wattage.
- Does the unit have a wiper or easy sleeve access? A wiper adds first cost and saves roughly an hour a month on a commercial pond.
- Where does the chamber sit in the loop — after mechanical filtration, full flow?
- Is the lamp and sleeve a standard size with local stock, or factory-only with a 2-week lead?
Ruibit's commercial koi pond packages pair the drum filter with a full-flow UV chamber and a variable-speed pump sized so the lamp sees the designed flow at the designed dose — the sizing is done on the drawing, not left to the installer.
Send the pond volume, fish load, feeding rate, and whether the pond gets direct sun, and we will return the UV wattage, flow arrangement, and a year-one running cost as part of the quote.
FAQs
How do I size a koi pond UV clarifier? By dose, not wattage. Use roughly 17–18 W per m³/h of flow. A 20 m³ pond at one turnover per hour needs about 350 W; 40 m³ needs two 350 W units or one 700 W.
What UV dose does a koi pond need? 20–30 mJ/cm² for green-water control, 30–50 mJ/cm² for pathogen control. Commercial sizing assumes a 0.3–0.5 efficiency factor for sleeve fouling and lamp ageing.
Should the UV run full flow or on a bypass? Full flow. A bypass at 20% flow delivers 20% of the dose budget. Also place the chamber after mechanical filtration, or suspended particles shield algae from UV.
How often should I replace the UV lamp? Every 8,000–9,000 hours, roughly one season of 24/7 operation. Output drops to about 60% by month 14, so running an old lamp wastes electricity.
Why is my pond still green with a correctly sized UV? Dirty quartz sleeve, aged lamp, or UV placed before the filter. A monthly sleeve wipe restores 40–60% of lost transmission.
