Koi Pond Filtration: The Complete System Guide
Drum filters, bead filters, UV clarifiers, bottom drains — everything you need to know about filtration for a SoCal koi pond.
Read Guide →What a UV Unit Actually Does
Low-pressure mercury lamps, the kind used in UV water treatment, emit almost all their light at a single wavelength, 253.7 nm[6]. That is in the UV-C band (100–280 nm), the wavelengths used for water disinfection because they damage DNA and RNA in cells[5]. It does not kill an organism immediately. It "introduces genetic interference that inhibits replication"[5]. Aqua Ultraviolet's lamp "alters or disrupts the DNA or RNA" of single-celled organisms such as algae, bacteria and protozoa[1].
UV only treats water that passes through the chamber, and leaves nothing behind in the pond[1]. The dose is irradiance multiplied by exposure time[8]. Push water through faster and each gallon gets a smaller dose. That is why the same lamp can be a sterilizer at one flow rate and only a clarifier at another. Aqua Ultraviolet says that to change what its unit targets, you "vary the flow rate" to "increase or decrease the amount of time the water is inside the body"[3].
Clarifier vs. Sterilizer: Same Lamp, Different Flow
Aqua Ultraviolet explains the difference. Run as a sterilizer, the unit keeps water "crystal clear" and adds bacterial control. Run as a clarifier, "the algae in your pond will be reduced, your pond will still have a green tint to it, unless you have 50-75% plant coverage or your pond is in full shade"[1]. Its clarifier sizing column is meant for ponds that are partly shaded and have about 50% plant coverage[1].
Aqua Ultraviolet's published ratings show the trade-off:
| Unit | Rated flow at a lower dose | Rated flow at a higher dose | Pond rating |
|---|---|---|---|
| Aqua Ultraviolet Classic 25 W | 1,200 GPH at 30,000 µW·s/cm², end of lamp life[2] | 400 GPH at 90,000 µW·s/cm²[2] | Sterilizer: 500–1,200 gal freshwater. Clarifier: up to 4,000 gal with 50–75% plant coverage[2] |
30,000 µW·s/cm² equals 30 mJ/cm². Aqua Ultraviolet prints it as "µw/cm²".
The same 25 W unit is rated for three times the flow at a clarifier-level dose as at a sterilizer-level dose: 1,200 GPH against 400 GPH[2]. Compare units on each maker's rating for the job you want, not on wattage alone.
What UV Fixes, and What It Doesn't
It fixes green water: free-floating algae, which Aqua Ultraviolet's units help control "at inflated flow rates"[3]. Aqua Ultraviolet expects clear water in 3–5 days with a correctly sized unit after the filter, or 7–10 days before it[1].
It doesn't fix:
- Algae growing on surfaces. "The green algae on the sides and bottom of the pond will not disappear… The UV does not control this type of algae"[1]. String algae grows attached to surfaces, so by that description UV won't control it either (our reading of the manual, not a manufacturer statement).
- Cloudiness that isn't algae. Murky water from koi stirring up the bottom is suspended dirt, not algae, and needs mechanical filtration (our guidance). See our filter guide.
Sizing: Why Wattage Charts Disagree
A watts-per-gallon number hides four variables, and manufacturers adjust their ratings for each:
- The dose you're aiming for. Clarifier ratings use higher flow, so a lower dose, than sterilizer ratings for the same unit[1][2]. Aqua Ultraviolet says its units control bacteria "at low flow rates" and help with free-floating algae "at inflated flow rates"[3].
- Flow through the unit. More flow means a lower dose[8], and makers assume different turnovers; Aqua Ultraviolet assumes the pond passes through every 1–1.5 hours[1].
- Fish load. OASE gives its Bitron C three ratings: a maximum pond size, a smaller size for ponds with fish, and a koi rating about one quarter of the maximum. For the 36 W model those are 40 m³, 20 m³ and 10 m³[11].
- Sunlight. Aqua Ultraviolet's clarifier ratings assume a partly shaded pond with about 50% plant coverage[1]. A pond in full sun needs the sterilizer rating.
The headline "max pond size" is usually the most generous of these ratings. For a sunny, well-stocked koi pond, the rating that applies is the manufacturer's koi, high-stock or sterilizer rating:
| Model | Lamp | Headline max pond | Maker's koi / high-stock / sterilizer rating | Max or rated flow |
|---|---|---|---|---|
| OASE Bitron C 36 W | 36 W | 40 m³ (≈10,570 gal)[11] | Koi: 10 m³ (≈2,640 gal)[11] | 30,000 l/h (≈7,925 GPH)[11] |
| OASE Bitron C 72 W | 72 W | 75 m³ (≈19,800 gal)[11] | Koi: 19 m³ (≈5,020 gal)[11] | 40,000 l/h (≈10,570 GPH)[11] |
| Aqua Ultraviolet Classic 40 W | 40 W | Clarifier column: 6,000 gal[1] | Sterilizer: 1,200–2,000 gal[3] | 2,900 GPH at 30,000 µW·s/cm²[3] |
| Aqua Ultraviolet Classic 57 W | 57 W | Clarifier column: 6,500 gal[1] | Sterilizer: 1,500–3,000 gal[4] | 3,200 GPH at 30,000 µW·s/cm²[4] |
Gallon figures marked ≈ are converted from metric (1 US gal = 3.785 L) and rounded. The last column mixes two kinds of figure: Aqua Ultraviolet's flows are rated at a stated dose[3][4]; OASE's are the units' maximum flows[11], and OASE describes a built-in "bypass system" for high flow rates[11]. Only a dose-rated flow can be used for step 2 below.
A practical sizing order:
- Work out the real flow your pump delivers at its head (see our filter guide).
- Pick a unit whose rated flow, at the dose you want, is at or above that flow (where the maker publishes one).
- Check that the maker's koi, high-stock or sterilizer rating covers your pond volume.
- If the pond is in full sun, use the maker's sterilizer rating, not its clarifier column[1].
What if the pump is bigger than the UV? All pump flow should go through the UV, as long as the total stays within its rated maximum. Aqua Ultraviolet advises against splitting flow[1]. Some installations use a bypass sized to the UV for pumps that exceed its capacity. Either way, the flow through the UV must stay within its rating.
Bulb Life: Why a Lit Lamp Can Be a Spent Lamp
Lamp output falls with age. Research on drinking-water UV systems, summarized by UNH, explains why. Causes include solarization (the lamp and sleeve turn more opaque), electrode failure and mercury plating on the lamp wall. Together these "may reduce output by 30% after 7,000 hours of use". Lamp life is about 7,500–8,000 hours, shorter with frequent starts and stops, and manufacturers recommend yearly replacement[6].
Pond manufacturers' intervals run from 11 to 14 months:
- OASE: at most 8,000 operating hours, about one pond season[10].
- Aqua Ultraviolet: 14 months; after that "the lamp may still light, but the UV has diminished"[1].
Our guidance: in Southern California's sunny climate, run the UV all year, so the hours add up fast; 8,000 hours is about 11 months of continuous running. Replace the bulb after 11–14 months of run time, depending on the maker; check your unit's manual. Don't touch new lamps with bare hands[1].
The Quartz Sleeve and Southern California's Hard Water
The lamp sits inside a quartz sleeve, and anything coating that sleeve blocks UV. Water hardness makes sleeves harder to keep clean[7].
Southern California's imported water is often hard. In Metropolitan Water District's 2025 report (2024 data), average hardness at its five treatment plants ranged from 84 to 272 ppm as CaCO₃, with three plants above 240 ppm[12]. Drinking-water research summarized by UNH found standard cleaning adequate below 140 mg/L, and those systems had wipers sweeping every 15 minutes[7].
Aqua Ultraviolet allows Lime Away or CLR, but no abrasives, which scratch the quartz[1]. Our guidance in hard SoCal water: check the sleeve monthly, as our filter guide recommends.
Where to Install It
Aqua Ultraviolet: "The UV should be installed after the filter on the return line"[1]. UNH's drinking-water guide concludes that "filtered water is the most appropriate application for UV disinfection"[9]. Installing before the filter works, but clearing takes longer, and debris from the pump can break the quartz sleeve[1].
Fit ball valves on both sides of the unit so it can be serviced (our guidance), and never submerge it[1].
Safety
- Never look at a lit bulb. Outside its housing a lit lamp can cause "severe sunburn and/or conjunctivitis"[1].
- Disconnect power and turn off the pump before servicing[1].
- Use a GFCI-protected circuit[1].
- Never run it dry or without flow. Aqua Ultraviolet warns that running too long "without water flow can cause extensive damage to the unit"[1].
- Handle lamps with care. They are low-pressure mercury vapor lamps[6].
UV, Medications and Beneficial Bacteria
"If good bacteria passes by the light it will be destroyed," says Aqua Ultraviolet, but bacteria colonizing the filter and pond surfaces are affected only if they float free through the UV[1]. For medications, Aqua Ultraviolet recommends "shutting down the sterilizer as the UV Radiation may affect the treatment"[1].
FAQ
Is a clarifier different hardware from a sterilizer?
Often it is the same unit run at a different flow[2].
How long until my green water clears?
Aqua Ultraviolet expects 3–5 days with a correctly sized unit after the filter, or 7–10 days before it[1].
Will UV get rid of string algae?
No. Algae attached to the pond's sides and bottom is not controlled[1].
How often should I replace the bulb?
After 11–14 months of run time, depending on the maker[1][10]; check your unit's manual.
Should I turn the UV off when I medicate my fish?
Follow the medication's directions. Aqua Ultraviolet recommends shutting the UV down while medicating[1].
Sources
- Aqua Ultraviolet, UV Sterilizer Instruction Manual (Classic Series, Twist Series, Swimming Pool & Spa Series), manufacturer PDF (2015 file)
- Aqua Ultraviolet, "Classic UV 25 Watt Series Units" product page aquaultraviolet.com
- Aqua Ultraviolet, "Classic UV 40 Watt Series Units" product page aquaultraviolet.com
- Aqua Ultraviolet, "Classic UV 57 Watt Series Units" product page aquaultraviolet.com
- University of New Hampshire WTTAC, Small Public Water System Technology Guide – UV Disinfection Educational Module, "UV Light" unh.edu
- UNH WTTAC, UV module, "Lamp and Sleeve" unh.edu
- UNH WTTAC, UV module, "Limitations and Concerns" unh.edu
- UNH WTTAC, UV module, "Microorganisms" (UV dose section) unh.edu
- UNH WTTAC, UV module, "Water" unh.edu
- OASE, Bitron C Operating Instructions, as hosted on ManualsLib manualslib.com
- OASE's published figures for the Bitron C 36 W and 72 W (as reproduced by a retailer; OASE's own site could not be reached).
- Metropolitan Water District of Southern California, 2025 Annual Drinking Water Quality Report (reporting period Jan–Dec 2024), table "Other Detected Constituents That May Be of Interest to Consumers" www-admin.mwdh2o.com