Choosing the right UV clarifier for your pond

Compared to some of the previous articles in this series, looking at choosing a pump, heater or filter, where I had my work cut out trying to condense the information required into a single article, choosing the correct UV for your pond is significantly more straightforward.

Firstly, compared to pumps and filters, there are fewer on the market to choose from (or be confused by), and the checklist for making the best UV choice for your pond is considerably shorter.

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What is the job of a UV unit in a koi pond?

Our battle with green water starts as soon as we mix the vital ingredients of water, nutrients, (phosphates + nitrates) and sunlight. Unfortunately, this cocktail will occur in every koi pond, resulting in the proliferation of algae which are particularly well adapted to rapid colonisation and division (That’s how crystal clear ponds can turn green over night). Even though most filters are fitted with mechanical media, it is not sufficiently fine to trap and intercept the microscopic single-celled algae that thrive in a pond. Besides it’s germicidal properties, UV light has been demonstrated to have an algaecidal effect on pelagic single-celled algae by causing them to flocculate into large clumps which can then be removed by mechanical means.

In fact, UV clarifiers are so good at producing a continual supply of green sludge that it is recommended that mechanical filtration should be ‘over-sized’ whenever UV units are installed.

Are there different types of UV unit?


Each brand of UV unit when viewed from the outside looks different. Some are made from ABS plastic, others are covered in a plastic outer shell or housing. More recently, stainless steel units have become available. These will inevitably prove to be more rugged that their plastic counterparts – but for a unit containing no moving parts, ruggedness is probably not that important.

Germicidal lamp:

The UV lamp is the business-end of any UV unit, irrespective of a unit’s external appearance. The vast majority of units use either Philips or GT UV bulbs – giving them comparable performance at the heart of each unit. Different manufacturers will boast better UV performance through different flow characteristics achieved through the quartz sleeve, but ultimately, they will use similar bulbs. Think of this as being like different car manufacturers choosing to use the same engine, but claiming different benefits on the strength of their different aerodynamic body shapes.

Quartz sleeve:

Like the UV lamp, most quartz sleeves (the barrier between the UV tube and the flowing pond water) are identical – with spare parts generally being universal sizes through the range of UVs. Oase have recently launched a UV with a different non-standard quartz sleeve that is required in their self-cleaning model (see later)


Just as we need different sized pumps and filters for different sized ponds, we also need a UV that matches our pond’s volume. The smallest is typically a 6 watt UV right up to a 55 watt unit, in steps of 8-10watts. Single UV units can also be coupled together to provide yet more UV output in a single unit, making UV’s very flexible in how they can be tailored to suit individual ponds.


You should also consider how straight forward any UV unit will be to plumb into your existing system. You will need to take note of your existing pipework and check that the UV unit that you are considering is supplied with the hose tails (for flexible hose) or adaptors (for rigid pipework). If not, these adaptors can add a significant cost to your UV purchase

What should you be looking for from a UV unit for a 2000 gallon koi pond?

1. Wattage. The rule of thumb for choosing the correct wattage UV is that 10 watts are required for 1000 gallons of water. So for a 2000 gallon pond, 20 watts of UV will be required. I also prefer to take the same approach with UVs as I do with a pump and filter – choosing one larger than is actually required. This gives you comfortable capacity in any extreme and demanding circumstances. The beauty of UV also means that unlike chemical treatments for greenwater, it cannot be overdosed. In fact, it is this desirable feature of UV that is seeing it be used increasingly in swimming pools – enabling the use of potentially irritating chlorine to be reduced.

2. Pump-fed or gravity-fed filtration. Koi keepers are generally divided on where a UV unit should be installed in relation to the filter. UVs have been shown to work in either configuration, but when opinion is split, it usually means there are good arguments for either preference.

a. Before the filter. A UV unit must run full bore. As most filters are pump-fed, it makes practical sense to site the UV between pump and filter.

b. After filter. Those koi keepers advocating a UV should be positioned after the filter do so on the grounds of superior UV performance and maintenance. They suggest that water passing through the UV unit after the filter will be cleaner and therefore allowing better penetration of the UV radiation. If you opt for an after filter position, then ensure that the UV runs at full bore.

What factors will influence performance?

The most important factor that influences a UV’s performance is its wattage in relation to the pond’s volume. One factor that UV manufacturers cannot control is how the UV will be positioned in relation to the filter as well as the flow rate that passes through the unit. There is a trade-off between how fast water passes through a UV unit and how frequently. The slower water passes through a unit, the better, giving a greater contact time in the tube, as long as the pond’s volume is turning over once every 2 hours.

Quartz sleeve. Slime, grime and limescale will tend to settle or form on the quartz sleeve, reducing the UV light’s penetration. The sleeve should be inspected and cleaned if required through the season (i.e. if you start to see a green tinge to your pond water). Oase’s Bitron UV has a built-in magnetic sleeve that will slide over the sleeve, clearing it of debris without having to disassemble the unit (just as a magnetic algae cleaner cleans the inside glass of an aquarium).

Bulb life. The output of UV radiation declines with the age of the bulb. The bulb should be changed at the start of every season to ensure your UV is at it’s strongest in the challenging warmer and sunnier summer months.

Costs and Benefits – Clear Water Guarantee

The vast majority of UV units are sold with a clearwater guarantee. As this is the prime objective of us buying and installing a UVc, it seems difficult to justify spending more than we need on a unit. Having said that, a little research soon shows that prices for a 25 watt UV can range from £80 up to £550! (and yet both are sold with the same clear water guarantee)

A UV can be a real asset to the pondkeeper by clarifying ponds and to some degree improving pond hygiene. When installed correctly, they offer excellent value for money, are easy to use and pose few risks to pond health (unlike chemical alternatives). However, they do pose some risks to the pondkeeper if safety guidelines during installation and maintenance are ignored and must be used in conjunction with rather than instead of other algae-removing methods (if you want to control blanketweed as well).

More information

I suggest you visit a koi dealer or aquatic store stocking several different UVs to choose from. Ask about maintenance costs (i.e. replacement quartz sleeves, bulbs, O-rings etc) and see which UVs are used on site.

How effective really is a UV?

As far as your pond’s overall ecology is concerned, a UV unit is a reactive treatment to a symptom of a nutrient-rich pond. It does not remove the nutrients from the water, leaving the gate wide open for other opportunistic algae to thrive unhindered and unshaded by green water. It is therefore not surprising to find that the occurrence of blanketweed in koi ponds has increased at the same time as UVCs have been used by koi keepers.

The clear water allows excellent sunlight penetration, coupled with the ready supply of nutrients that are no longer being used by green water can produce phenomenal blanket weed growth.

So UVCs can cause as well as solve algae problems in a pond. A UVc can be used very effectively in conjunction with pond treatments that consume or bind-up algae-promoting nutrients, which prevent the re-growth of other algae such as blanketweed.

Box out: Things you need to be aware of and ask about when buying a UVc

a. Your pond’s volume – and choose the wattage accordingly

b. Where you intend to install it – and the plumbing and electrical implications

c. Look out for a unit carrying a clearwater guarantee

d. Establish the annual bulb replacement costs, and how easy it is to replace.

Box out: UV unit performance

a. Bulb wattage

b. Flow rate and contact time

c. Condition of quartz sleeve

Box out: Unit Maintenance

a. Quartz sleeve removal and cleaning. Removal of O-rings and couplings to release quartz sleeve so it can be cleaned. If limescale build up is severe, consider replacing it.

b. Bulb replacement. Replace at the beginning of each season, being sure to check that the replacement bulb illuminates once installed.

Kill blanketweed and string algae.