Many pond keepers combat green water problems with an ultra-violet (UV) filter. A UV filter is simply a section of pipe with a UV bulb in the center. Pond water flows around the UV bulb. As algae in the water goes by the UV bulb, it gets sunburned to death and your pond water gets cleared of algae.

One of the most commonly used UV filters are those made by Rainbow. A major shortcoming in the Rainbow design is that it greatly restricts water flow. With a small investment of time and PVC plumbing parts, you can build a new housing that has very little resistance to water flow.

The diagram shows how to construct your own UV housing. These plans make use of the quartz tube, UV bulb, and ballast in the original Rainbow filter. If you're considering building your own housing, but don't already have a Rainbow UV, you may want to simply buy a whole Rainbow UV filter and discard the housing. Buying just the pieces needed will cost about the same as a whole filter.

The plans show the main housing made from 3" PVC, but you could substitute 4" if desired. Start by getting all the needed parts, including the guts of a Rainbow UV.

There are two parts to be built - the upper section which connects to the quartz bulb sleeve, and a lower sleeve support.

Start construction of the upper assembly by cutting a ½" coupler in half. Epoxy the coupler piece into the center of a 3" x 1.5" bushing. Put duct tape across the opening to reduce the amount of epoxy needed and to hold the ½" coupler in place while the epoxy sets. Use an epoxy that has at least 30min. working time. You'll be able to see a blue glow of the UV through the epoxy when the filter is in operation.

The lower sleeve support is simply a piece of ¾" PVC pipe that is cut to fit inside the 3" PVC and has a hole in the middle for the quartz bulb sleeve to rest in. If you have a tight fit between the ¾" pipe and the 3" pipe, you can secure the connection using PVC cement. If the fit is a bit loose, use epoxy.

Handle the quartz bulb sleeve very carefully - its very brittle and easily broken.

Remember to replace your UV bulb every spring. Even though it looks like it's still working fine, the amount of UV produced drops significantly after a few months of use.