Pond keeping should be enjoyable and relaxing not a frustrating battle with green water. Ultraviolet sterilizers are here to stay. If you are thinking of purchasing a unit you will need to consider the capabilities and the limitations.
U/V sterilizers will keep your water clear of planktonic algae. This means that a new pond does not have to go through the green water stage. If you already have an established pond an ultraviolet system will clear an algae bloom within one to two weeks. The unit will only work, provided it is of good quality and is sized properly for the water volume in your pond and it is properly installed.
U/V sterilizers will not affect the hairy stringy types of filamentous algae that adhere to the walls of the pond. The ultraviolet rays only kill the algae which passes through the unit.
U/V sterilizers will allow a new pond to become established and balanced sooner than without such a device. New ponds need to become established with beneficial bacteria in the biofilter, and a smooth coat of dark algae about 1/4 inch thick on the walls and floor of your pond. These two types of organisms are necessary for a healthy pond. The bacteria and smooth algae are slow growers especially in cold water The planktonic algae on the other hand can bloom in two days. The planktonic algae in full bloom inhibits the bacteria and wall algae by competing for nutrients and blocking out sunlight. This makes the planktonic algae "king of the pond" and can dominate indefinitely. Shallow ponds in full sunlight and overcrowded ponds can be subject to quite lengthy bouts of "pea soup". The ultraviolet rays kill the planktonic algae. By removing the planktonic algae, your pond is able to become balanced more quickly.
Ultraviolet sterilizers will not be very effective at controlling disease within the pond. These units are more capable of controlling disease within a smaller aquarium. To control disease with a ultraviolet system the water must pass through the unit very slowly. Microscopic bacteria and parasites may need a fifteen second exposure time to affect a kill. The planktonic algae is very light sensitive and can be inhibited at much faster flow rates. To effectively control diseases within a large body of water with a ultraviolet unit the entire volume of water must be passed through at a slow rate. The trouble with this method is that the pathogens are reproducing within the pond faster than an ultraviolet system can kill them. An ultraviolet system will not kill any pathogens on the skin of the fish. A very large and expensive system would be required to control disease within a pond.
Good quality ultraviolet sterilizers will be constructed with an external housing made of PVC or other inert material and hued with waterproofed caps. When used outside, a waterproof housing should cover the ballast. The unit like all electrical parts used on your pond should be UL listed.
Ultraviolet rays are in the extremely short wavelength of the light spectrum and can only penetrate a few inches into the water chamber. Ultraviolet units for ponds are only about three inches in diameter The bulb itself needs to function at a certain temperature to produce the peak amount of ultraviolet rays and penetration into the surrounding water area. A quartz sleeve between the water and the bulb, keeps the bulb at the proper temperature and does not interfere with the light penetration.
Proper installation of the unit consists simply of adding a bypass line to an existing pressurized water line from your pond pump or allow better light penetration. Add two PVC tees and a ball valve to your existing line and connect the unit to the assembly. Use the ball valve to control the flow through the unit. Do not exceed the manufacturers suggested flow rate.
Ultraviolet sterilizers have their advantages but are not without limitations. New ponds as well as ponds that have a persistent problem with green water will benefit from a ultraviolet system. Older more established ponds with adequate filtration may not see a noticeable difference. Use ultraviolet systems to complement a good filtration system. A unit will assist in achieving maximum results and appreciation of your pond.