UV Sterilizers & Clarifiers

Green water is single celled algae suspended in the water column, and no filter catches it because the cells are too small. A correctly sized UV solves it another way: water passes a UV-C lamp, the cells clump, and your filter finally gets them. Most ponds clear within three to five days.

Complete units from Aqua Ultraviolet, Teton, Tetra and PondMax, plus every replacement lamp, quartz sleeve, transformer and seal. Size for sterilizer duty rather than clarifier duty on a koi pond, plumb it after the filter, and replace the lamp on the calendar rather than when it stops glowing. It keeps lighting long after its UV-C output has gone.

Clear quartz sleeve, a UV-C lamp tube and a stainless steel sterilizer housing on a stone ledge beside a koi pond, used to clear green water and control waterborne pathogens.

Size it right the first time

Manufacturers' own numbers, the same charts behind our pond sizing calculator.

UV: size for sterilizer duty, not clarifier duty

The same unit does two jobs depending on how fast you push water through it. Run it slow and it is a sterilizer: clear water plus control of free-floating bacteria and parasite stages. Run it fast and it is a clarifier, which handles a much bigger pond but only knocks algae back. Koi ponds want sterilizer duty. The figures below are Aqua Ultraviolet's own published chart, and unusually they are calculated at end of lamp life rather than on a brand new lamp.

Swipe the table to see max flow →

Pond volumeAqua UV ClassicMax flow through the unit
Up to 200 gal8 W642 GPH
200 to 500 gal15 W700 GPH
500 to 1,200 gal25 W1,200 GPH
1,200 to 2,000 gal40 W2,900 GPH
1,500 to 3,000 gal57 W3,200 GPH
2,200 to 4,400 gal80 W or 114 W3,678 to 3,900 GPH
4,400 to 6,000 gal120 W4,080 GPH
6,000 to 8,500 gal160 W5,400 GPH
8,500 to 13,000 gal200 W6,600 GPH
13,000 to 17,000 gal240 W7,200 GPH

If your pump moves more water than the unit's max flow, tee off a bypass rather than sizing down. Water travelling too fast does not get enough exposure to do anything. Lamps are a 12 to 14 month consumable: we stock every lamp, sleeve and transformer.

Worth knowing before you buy

Ground rules and answers from the ponds we run every day in San Jose.

Never run the pump dry, or the UV without flow

Mechanical seals fail in seconds without water, and a UV lamp running in a dry chamber overheats the quartz sleeve. Wire them so they start and stop together.

Turn the UV off while you medicate

UV-C degrades most treatments, and activated carbon strips them out entirely. Bypass both for the duration of a course, then bring them back online.

Common questions

My water is green. Do I need a UV?＋

If it is pea-soup green and you cannot see your hand a few inches down, that is suspended single-celled algae and yes, a correctly sized UV is the reliable fix. Most ponds clear within three to five days once it is running. Plumb it after the filter so the water reaching it is already clear, because UV-C cannot penetrate dirty water.

What a UV will not do is stop the algae coming back if the underlying cause is untouched. Green water is fed by nutrients from fish waste, overfeeding and sunlight, so a UV is a control, not a cure.

I have stringy algae all over the rocks. Will a UV fix that?＋

No, and this is the most common misunderstanding we hear. String algae is filamentous and it clings to surfaces, so it never passes through the UV chamber. Running a bigger UV will do nothing for it.

String algae is a nutrient problem. Pull it out by hand where you can, cut back on feeding, keep up water changes, and use an algaecide or a barley product to buy yourself time while the real fix takes hold. Ironically, ponds with excellent UV often get more string algae, because the free-floating algae is no longer competing for the same nutrients.

I am not sure what I need. Can you just tell me?＋

Gladly, and this is genuinely the fastest route. Send us your pond volume, your depth, roughly how many koi you keep and how big they are, your plumbing size, and the height from the pond surface up to your filter return. That is everything we need to size a system properly.

We have been building and maintaining koi ponds from San Jose since 1998, and we run this gear on our own systems. We would rather talk you into the right pump than sell you the biggest one.

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