The best way to prevent the pond water to turn into to the greenish disgusting color caused by planktonic green algae, an ultraviolet sterilizer or a UVC (ultraviolet clarifier) is what you need.
Aside from the fact that UVCs and sterilizers are the most effective when it comes to eliminating the existence of suspended algae, it is the safest to use in ponds. They cannot be overdosed by UVC light unlike chemical algaecides.
The wattage specification of each UVC and UV sterilizer that you will use will depend on the volume of the pond water, the number of fish in the pond and the amount of sunlight that hit the water system.
It important that you may put the pond pump and UVC or sterilizer on the same electricity circuit so that when the power goes out, all of the gadgets in the same circuit will stop working. This will prevent the UVC from working without water circulating through it. If in case it is connected to a separate circuit and continues to work, the tendency is that deposits will begin forming on the quartz tube surface due to water evaporation.
The best way for you to maximize the function of UVCs and UV sterilizers in your pond is to make sure of the following:
1. The volume of the pond water matches the wattage of the UVC that you chose to use. For example, if the pond has about 250 gallons of water then its match would be a 4 watt UVC or UV sterilizer.
2. The pond should be getting enough sunlight for the UVC to work at its best.
3. If in case your pond is in a shady area and doesn't contain many fish then a 4 watt UVC would work well in a 700-gallon pond water.
4. If there are a lot of fish in the pond and there is enough water, go for a UVC or a sterilizer that has a higher wattage.
The pointers mentioned above are just guidelines especially for those who are just beginners in pond maintenance. Always keep in mind that when you are hesitant as to how many watts to purchase, always go for the higher number. This is to make sure that the UVC that you will buy won't be useless.
There are some cases wherein the power of the UVC or UV sterilizing light doesn't match the need of the pond although it matches the water volume. There are actually many reasons why it happens. Warm climate can cause you to change your UVC to a higher wattage since there is too much sunlight that enters the pond. The number of fish also affects the effectiveness of the UVC and sterilizer. The more fish you have, the higher the watts of your UVC should have. In other cases where the pond is maintained at 20C (68F) in temperature, a higher UVC wattage must be used.
In general, for you to maximize the capability of an ultraviolet clarifier or sterilizer, always check these things: sunlight, temperature and fish.