A Guide to Modern Koi Pond Construction

If you have green water then the ultra violet sterilisers will in most cases cure the problem. However, your filter should be big enough to solve the green water problem, and you should not rely on the UV for this reason.
Even with a good working filter system it is still an advantage to fit a UV unit as it will also kill bacteria, and this is now the main reason that most koi-keepers now fit one.

This photo shows two 55 watt TMC Pro Clear Advantage U.V. Clarifiers in use on a pond.

These are now very popular and one can be used on ponds up to 6,000 gallons.

All these units use a 30 watt or 55 watt tube. Smaller ones are available, but will not be large enough for the type of ponds we are looking at here.
It is generally considered that 30 watt is sufficient for ponds up to 3,000 gallons, and 60 watt should be used for 6,000 gallons. You can add as many as you wish, but they will be used for at least 6 months in an unheated pond, and longer on a heated pond so the cost of electricity has to be taken into account.
All these units have a quartz sleeve, and the tube fits inside, and can be removed without stopping the water. It should be borne in mind that the tube is about 37 inches long, and needs to be moved from either end. If the pump chamber is above ground then you can arrange an opening to be left in the wall. If you cannot do this then it will be necessary to put couplings in the pipework so that they can be unscrewed and the whole unit can be lifted out.

There is a lot of discussion as to whether the UV should be switch off in the winter, but most people with heated ponds tend to keep them on for 12 months of the year.

NOTE. The tube must be changed every six months as it will not work efficiently after that time, although Philips now have a tube with a life of 12 months The unit is usually switched off in the winter months.