One of the fastest growing products for a Koi pond is the ultra-violet (UV) sterilizer. The UV sterilizer is a device which uses ultra-violet light to kill micro-organisms in water. UV light bulbs look like a fluorescent light bulbs. Ultra-violet disinfections have been used for many years in hospitals and in water purification. There are even units made for sanitizing spa and hot tubs. The aquarium hobbyists have UV sterilizers available for delicate aquarium set-ups. The Koi hobby has known about UV for over 10 years, but have not accepted the technology until now. The reason is, there is some confusion as to what to expect. A UV unit is not meant to replace a bio-filter. You still need a bio-filter for removing the ammonia wastes. You should not expect the UV unit to sterilize your pond. There will be bacteria on and in your fish, in the biological filter bed, and on the walls and bottom of the pond. What you can expect is to control harmful bacteria, algae, and other waterborne micro organisms by reducing their numbers. The trick is not to destroy all the organisms in one cycle. But, to expose the unwanted organisms to repeated small doses over a period of time. With this in mind, you will need a 1900 - 5700 lit per hour flow rate for a 15 - 50 watt UV unit. If your flow rate is greater, you can install a "tee" before the UV sterilizer and bypass the excess flow around the unit. When using a "tee", have the main flow directed straight into the UV unit and the bypass coming off the existing straight pipe. That way you will get maximum flow through the UV unit. Since some of the UV sterilizers are designed for other applications, it is important to know what to look for:

The unit should be designed for outdoor use or be provided with a suitable enclosure. Electrical supply should be protected with a GFI.

The unit should have an effective and efficient design. UV lamps that are surrounded by water are much more efficient than a lamp suspended over water. UV light does not behave like visible light. For example, it does not pass through normal glass or transparent plastic, and it reflects badly from most surfaces (even mirror finishes).

The UV sterilizer bulb must be surrounded by a quartz glass sleeve. A UV bulb normally operates at 23o C or above. The special quartz sleeve allows the bulb to operate in colder water at its normal temperature for maximum performance. UV bulbs have a limited effective life of 7000+ hours. Bulbs should be changed at least every year.

With efficient designed UV units, you will need about 10 bulb watts for every 3800 - 4500 lit of pond. For stubborn algae problems, you will need more light because algae is more resistant to UV light and UV does not penetrate cloudy water and looses efficiency.

The UV unit should be installed so that the bulb is surrounded by water, no air pockets.
In conclusion, UV sterilizers do work, but they are not inexpensive. Be aware of poorly designed equipment. Usually a normal sterilizer unit looks like a 3 inch white PVC pipe with inlet/outlet connections on the side at each end and a UV tube running through the centre. The ends of the tube (UV bulb) will be sealed with o-rings and the wiring connections covered with rubber boots.