Pond UV clarifier size calculator
A UV clarifier clears green water by exposing free-floating algae to UV-C light. Size it by pond volume and fish load, then check the flow through it suits the unit.
Sizing rules
- Clarifying, light stock: about 1 W of UV-C per 75 US gallons (285 L).
- Clarifying, koi or heavy stock: 1 W per 50 US gallons (190 L) — more food, more nutrients, more algae.
- Sterilizing (to kill free-swimming parasites and bacteria): roughly double the wattage, with slower flow.
The result is rounded up to a common lamp size (5, 9, 11, 13, 18, 24, 36, 55, 75 or 110 W). Flow matters as much as watts: water that rushes past the lamp isn’t exposed long enough. Keep the pump flow within the range the UV manufacturer lists, and replace the lamp every season — UV-C output fades long before the tube stops glowing.
UV treats green water, not string algae, which grows on surfaces. It also does nothing for the cause: too many nutrients and too much sun. Plants that cover a third to half of the surface help both.
Questions
What size UV do I need for a 1,000-gallon pond?
About 13.3 W for goldfish (1,000 ÷ 75), so an 18 W unit, and 20 W for koi (rounded up to 24 W).
Should a UV clarifier run all the time?
During the growing season, yes — algae multiply fast. Many keepers switch it off in winter and fit a new lamp in spring.