koi pond filter

To keep the water in my pond crystal clear, I designed a filter system to match it.

The filter consists of a few different steps.

First there is the skimmer. A skimmer is a floating ring of plastic around a tube, which only has a little buoyancy. When you pump the water from the center tube, the ring will sink a little and water starts pouring over the top. As water pours over the top, the water level in the center rises again, increasing buoyancy. This will cause a balanced situation, in which only the top layer of water is pumped away. All leafs and dust and floating debris will collect in the center tube and leave the water surface clear. The center tube has enough room to house some filter material and acts as pre-filter. This part is easy accessible and has to be cleaned out regularly.

Then we have the pump. This pump is 60W and pumps about 5200L/H (twice the volume of the pond). It pushes the water through a pressure filter with Japanese mats and a 12W UV-light. After that it goes through a blue barrel filled with PVC ribbon.

I learned about the PVC ribbon trick on the Internet. It acts as housing for bacteria and is an effective bio-filter. You can buy bio carriers in the store, but it is much cheaper to make yourself when you happen to own a lathe. I started with 14 x 4m of 19mm PVC electrical tubes. I cut them up in manageable sizes and but them on my lathe. To keep them steady I used a steel rod as center. The lathe cutter can move at a preset pace and slowly eats the PVC tube. The thinnest manageable ribbon I could make is about 0.11mm. With 14 lengths this gives an effective surface area of about 120 square meter. This is enough to fill the 200L barrel. The perfect way to keep the ribbon from tangling wile spinning is using a vacuum cleaner to suck it up right from the cutter head. I have a big Vacuum with a large container. It can hold a lot of ribbon. Beware of static electricity though.

After that the water goes through the plantfilter/ wildlife area and flows back into the main pond over a waterfall or an internal tube. This part is also insulated with 10mm styrofoam to prevent freezing in the winter.

To monitor temperature, I added a standard temperature sensor with a cable. This way I can effectively measure the outside temperature and the water temperature. The filter compartment is covered with a sheet of wood, matching the rest of the build. This panel is easily removed when maintenance is required.

And some mechanical construction photos: