A UV sterilizer can be a very useful addition to any pond or water garden. The UV will do much more then just remove single cell algae (green water). Of course it will do that very effectively, it will also reduce harmful bacteria and help control parasites.
The size of the UV is very important. Choose the size of the UV based on the amount of water you want to flow through it. This is usually determined by the size of pump, but if you want a larger pump and a smaller UV, then you can allow only some of the water to flow through the UV by utilizing some sort of bypass. The reason for this is if the water passes through the UV too quickly then the unit will not be as effective. The microscopic organisms must in contact with the UV radiation long enough for them to be destroyed.