One of the most contemporary products for a Koi pond is the ultra-violet (U.V.) sterilizer. A U/V light easily kills planktonic algae (or pea soup as most pond keepers usually call it). U.V. light bulbs look like fluorescent light bulbs. Ultra-violet disinfecting has been used for many years in hospitals and in water purification and it also cleans up ponds and aquariums.
As effective as U.V. lights are for killing algae they will not clean up everything. However an ultraviolet light will not kill disease pathogens or parasites such as the Aeromonas bacteria. Essentially, the parasites can grow faster than the U/V light can kill them. Many fish owners use a combination of chemical and U/V light therapy to keep their ponds clean.
Pea soup algae on the other hand are light sensitive and are controlled with ultraviolet light treatment. However, the U/V will have a minimal impact on the growth of stringy algae attached to the pond walls.
A U.V. 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 U.V. 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 microorganisms by reducing their numbers.
Before selecting a U.V. light sterilizer for your fish pond you need to keep some factors in mind. First off make sure that the U.V. lamps can be immersed in water as they are much more efficient than the model of U.Vl lamp that is suspended over the water. The unit must also be surrounded in a quartz glass sleeve and the bulb should operate in colder water without shattering. A high quality bulb will have a life of 7,000 waters. To be sure of the water quality the bulb should be changed at least one a year.
One drawback is that U.V. sterilizers can be expensive. 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 U.V. tube running through the center. The ends of the tube (U.V. bulb) will be sealed with o-rings and the wiring connections covered with rubber boots. If the unit is cheap it might be poorly designed or meant for an aquarium and not a koi pond!