Pond keeping should be enjoyable and relaxing, not a frustrating battle with green water. Ultraviolet systems are here to stay. If you are thinking of purchasing a unit you will need to consider their capabilities and their limitations.
Ultraviolet 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 in one to two weeks. The system only works if the unit is of good quality, sized properly for the water volume in your pond and correctly installed.
Ultraviolet sterilizers will not affect the 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.
Ultraviolet sterilizers will allow a new pond to establish and balance sooner than without such a device. New ponds need to establish beneficial bacteria in the biofilter and a smooth coat of dark algae about 1/4" 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 it can dominate indefinitely. Shallow ponds in full sunlight and overcrowded ponds can be subject to quite lengthy bouts of ?pea soup?.
Ultraviolet sterilizers are not very effective at controlling disease within the pond. These units are more capable of controlling disease within a smaller aquarium or pond. To control disease with an ultraviolet system the water must pass through the unit very slowly. Microscopic bacteria and parasites may need a fifteen second exposure 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 an 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 then an ultraviolet system can kill them. An ultraviolet system will not kill any pathogens on the skin of the fish.
Look for good quality ultraviolet sterilizer constructed with an external housing made of PVC or other inert material and fitted with waterproof end caps. When used outside, a waterproof housing should cover the ballast. The unit used on your pond should be UL listed.
Ultraviolet rays are short wavelength in the light spectrum and so, 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 penetrate 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 includes the addition of a bypass line to an existing pressurized water line from your pond pump or prefilter. Cleaner water will 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 tin achieving maximum results of your pond.