Ultraviolet (UV) light to control green pond water

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Last updated 30th May 2014

The problem of Koi/other fish and green pondwater
Algae-removing ultraviolet units for garden ponds
UV effect on blanket weed in garden ponds
UV effectiveness on green pond water
UV units for garden ponds
Maintenance of UV units for garden ponds
Five top tips for choosing and using a UV unit for a garden pond

The problem of Koi and green pondwater

Water plants are the answer to green water, even if there are fish in the pond, but because of the design of Koi ponds, and the nature of Koi carp, plants are much more difficult to nurture, so green water is a long-term problem for Koi keepers.

Tiny single-celled algae multiply rapidly on warm bright days to create the green effect and without competition from plants for light or nutrients, the effect stays. Then the murkiness blocks light to any submerged oxygenating plants, which then grow very slowly, so have little effect on the greenness.

The large population of algae depletes dissolved oxygen concentrations (DOC) and produces carbon dioxide, which in turn can make the water more acidic at certain times of the day. This fluctuation in pH in turn affects the chemical composition of the pond water and is certainly bad news for Koi.

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Algae-removing ultraviolet units for garden ponds

Although the algae that create green water are too small to be filtered out, all is not lost since the advent of ultraviolet (UV) light treatment and ultraviolet clarifier (UVC) units.

UV units are available as separate items or integral to a filter system, but however they’re supplied, the principle is the same. What you’ll find is a tubular UV lamp sealed in a clear protective tube. Pond water is directed over and around the UV lamp at the recommended flow rate so that the UV light can disrupt the algal cells enough for them to clump together. These clumps can then be filtered out.

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UV effect on blanket weed in garden ponds

UV treatment won’t work for blanket weed, which is a filamentous form of algae. In fact, this could become worse as the green murk clears and more light can reach the pond’s depths. Although in the long term, the single-celled spores produced by blanket weed will be affected by the UV beams, thereby curbing growth.

For immediate reduction of blanket weed, choose one of the specific products on sale, for example from Pondkeeper.

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UV effectiveness on green pond water

A UV unit should have an effect on existing green water within three weeks. If not, then you should check that the flow rate of water through the unit matches the recommended rate and can deal with the pond’s volume.

Manufacturers tend to assume that a pond contains a few small fish plus plenty of plants, so Koi keepers need to fit a larger UV unit than often recommended (or combine a couple eg from Bradshaws or Pondkeeper) to deal with the specific demands of a Koi pond.

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UV units for garden ponds


Most stand-alone UV units are supplied with fittings to mount to a vertical or horizontal surface so that you can install them in the most convenient location externally. But if you don’t have external pipework, there are units that provide the option of being submerged in the pond if required.

Some units are designed to fit on top of the black-box type of filter used to keep garden ponds clear. Others are designed to deal with the larger gallonage and higher stocking levels of enthusiasts’ Koi ponds.

In addition to price, look at specific features, recommendations, claims for longevity and ease of changing lamps, and ask fellow water gardeners for their experiences, especially in maintenance.

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Maintenance of UV units for garden ponds

Once the UV lamp’s protective sleeve becomes dirty, the pond water and algae won’t be exposed to the same level of UV light. So regular removal of dirt and limescale is essential for the optimum operation of your UV unit. The difficulty with cleaning is that the protective sleeve can be very delicate and easily shattered. So buy a spare bulb when you buy the unit.

Limescale build up can be controlled with a magnet, and these may be fitted already, or you could add a limescale/blanket weed magnet to another part of your system.

For cleaning dirt off the sleeve, you’ll have to remove it first (and risk breakages), unless you choose particular models fitted with cleaning brushes that you can operate externally. And remember to read the manufacturer’s instructions carefully before proceeding.

With time, the effectiveness of the UV bulb with diminish, so replacement becomes necessary.

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5 top tips for choosing and using a UV unit for a garden pond

  • UV transmission. The clear tube protecting the UV lamp from the water should let through as many UV rays as possible. Check what percentage of UV rays reach the water.
  • Ease of fitting and maintenance. Fit the UV unit where it can be maintained in situ. If this isn’t possible, use hand-tightened fittings for ease of removal from the system. Easy-to-remove hosetails make removal easier.
  • Electrical safety. Always use a residual current device (RCD) in conjunction with mains electrical equipment in the garden to protect against electrocution.
  • Installation. Install UV units so the water flows past it before entering the biological filter in a pump-fed system; install after the biological filter and pumped outlet in a gravity-fed system.
  • Flow rate. The maximum flow rate is usually 35 to 50 per cent of pond volume. If the flow rate necessary for the filters is too great for the UV unit, then use a T-piece to divert the surplus and bypass the UV unit. But, if you allow too great a flow past the lamp, it won’t be effective at all because the water and the algae won’t be exposed to the UV light rays for long enough.

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