Pond UVc – Algae clarifier

It is not even 30 years since UVCs first became available as a mainstream piece of pond hardware – and so great has been their success and influence on our hobby that a UVc is now widely regarded as a ‘must-have’ on a pond keeper’s shopping list. Why have they proven to be so successful, and do you really need one?

Q1. Why might my pond need a UVc?

If you own a pond, then it is highly likely that your pond has suffered from green water at sometime through the season. Greenwater is caused by a suspension of microscopic algae that bloom under specific favourable conditions, causing the appearance of your pond to deteriorate into a pea green soup in only a couple of days.  A UVc (which stands for Ultra Violet Clarifier) works in conjunction with a pond filter to clear your pond of greenwater. Microscopic algae cells that before were too small for a filter to remove, now clump together when exposed to the UV light into particles that can be removed by a conventional filter.

Q2. I thought my biofilter alone would be sufficient to keep my pond water clear.

A biofilter’s primary function is to break down and detoxify the by products of digestion that fish excrete into the pond. However, a mature biofilter in conjunction with a balanced and well-planted pond will often be sufficient to keep pond water clear. The benefit of fitting a UVc is that it guarantees clearwater in even newest of ponds, with most UVc manufacturers now offering a ‘clear water guarantee’.

Q3. How does a UVc work?

Ultra violet light (or more correctly radiation) is found between the visible blue end of the electromagnetic spectrum and X rays. UV light fills a broad band within the spectrum from 13nm to 400nm (nanometres).

The germicidal UV light falls between 200 and 295nm with most lethal rays being at 254nm. At 254 nm, UV radiation is at its most lethal. It is an indiscriminate living tissue killer (i.e. it kills both pathogenic and non-pathogenic micro-organisms). For this reason, under no circumstances must the bulb be viewed directly or indirectly by the naked eye as it can cause tissue damage.

Compared to other pieces of pond equipment, UVCs are relatively straightforward in their design. It is merely an electric bulb seated within a water-tight housing. In fact, it is the UVCs simplicity of construction and design that makes them so easy to install (even to retrofit into an existing pond set-up), making them almost as standard in a garden pond as a pump and filter. Germicidal UV light causes these elusive cells to clump together into larger particles that can then be removed by a conventional filter, thus solving the green water problem.

Q4. Are they easy to install?

A UVc is very easy to install, requiring basic plumbing and electrical DIY skills. All that’s involved is cutting the hose between your pond pump and filter and inserting the UVC (which will most likely be fitted with two tapered hose tails for ease of installation). It will then simply need connecting to the same electrical source as your pump to ensure that the 2 units run simultaneously. Remember to use waterproof connectors and junction boxes.

A UVC should be pump-fed, positioned either between a submersible pump and a filter or after a dedicated UV pump and the return to the pond.  This will ensure full bulb coverage improving the efficiency of the unit.  Some pond filters are sold with an integrated UV, making their installation even more straightforward.

A UV clarifier should run continuously from spring to autumn to have the desired effect on green water. Units start at 4W for the smallest ponds, right up to 55W bulbs which are rated to clear a 10,000 gallon pond.  For even larger ponds, a series of UV’s can be installed to cater for the output required to guarantee clear water.  The  wattage of even the larger UV bulbs is less than the average household bulb, making them very economical to run.

Q5. What about maintenance?

A UVC should be turned on in the spring, having been fitted with a new bulb (which will last for 12 months), requiring minimal maintenance through the season.

The quartz sleeve may need cleaning to remove sludge or mulm and in extreme cases, may need replacing if it has accumulated a coating of stubborn lime scale.

Immediately after fitting, you will notice an increase in the amount of organic sludge being deposited in your filter. This is the flocculated microscopic greenwater cells that have now been removed by your filter.

Because of this extra loading produced by the UVc, it is recommended that mechanical filtration should be ‘over-sized’ whenever UV units are installed.

You will have to keep on top of cleaning your filter as your foams or other mechanical filter media will become overrun more frequently than before.

Q6. How much do they cost to buy and run?

UVcs are available as single units (if you already have a filter) or as combined filter/UV units (if you are building a new pond).

Single UVcs:


UVC for 600 gallon pond         –         £50.00 (6W)

“         10,000 gallon pond      -         £170.00 (55W)

Replacement bulbs     (6W)     -         £7.50

“             “        “     (55W)      -         £25.00

Running Costs:

For a 6W UVc for one season. Assuming 1Kwh = 10p

= £3.02

Q7. Are there any drawbacks of installing a UVc?

It is difficult to list any serious disadvantages of using a UVC and as a result, they can be installed to carry out their clearing role with great confidence.  It is not possible to overdose with UV and its action is limited to the water which passes beneath the bulb.  Furthermore, a UVc is likely to improve fish health  by reducing the number of pathogenic organisms (bacteria, viruses, fungi and parasites) in a pond. Consequently a UVC is very environmentally friendly and will not interfere with other desirable plant growth in a pond.

However, as far as pond 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 your pond water on which the greenwater thrives, 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 incidents of blanketweed in garden ponds has increased at the same time as UVCs have become available to pondkeepers.

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 blanketweed treatments that consume or bind-up algae-promoting nutrients, which will prevent the re-growth of other algae such as blanketweed.

Blanketweed – how to kill it in your pond.

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