Killing green pond water with a UVc

One of the most widespread problems encountered by koi keepers is green water. A suspension of microscopic algae will bloom under specific favourable conditions, causing the appearance of the pond to deteriorate into a pea green soup in only a couple of days.

Green water can be one of the simplest pond problems to cure. UV clarifiers are safe, cheap to run and easy to install and are a guaranteed way of preventing algae from causing unsightly green water. 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. UVCs are so successful at their job that no pond should be without one.

How the UV system works:

A UVC should be installed so that it is 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.

The UV lamp is sealed away form the water within a clear quarts sleeve. As water is pumped through the unit, the UV light causes the exposed algae cells to clump together, resulting in crystal clear water.

A UV clarifier should run continuously to have the desired effect with the UV out put of the bulb largely determined by the power consumption of the bulb (described in Watts). Units start at 4W for the smallest ponds, right up to 55W bulbs which are rated to clear a 10,000 gallon pond. For larger volume ponds, multiple UV’s can be installed to cater for the UV output required to guarantee clear water. As the power consumption of UVCs from even the largest of koi ponds are less than that of a household bulb, there is no need to be concerned with a UVCs running costs. 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. Consequently a UVC is very environmentally friendly and will not interfere with other desirable plant growth in a pond.

Perhaps the most widely acknowledged problem with using a UVC is that blanketweed can get out of control in a crystal clear pond. Spoiled with excellent growing conditions and a ready supply of nutrients no longer utilised by green water, blanket weed growth can prove to be quite phenomenal. This can be controlled by using phosphate removing additives that will starve blanketweed of its necessary nutrients.

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

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. Most UVCs are sold with universal tapered hose connectors which will fit most hose diameters, which are fixed in place with jubilee clips.


UVC for 600 gallon pond – 50.00 (6W)

10,000 gallon pond – 170.00 (55W)

Replacement bulbs (6W) – 7.50

(55W) – 25.00

The UVC has brought crystal clear water within the reach of any DIY pond owner at a reasonable price. Very straight forward to install, a koi pond is now regarded as being incomplete without a UVC.

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