I therefore decided to research and build a biological filteration system for the existing pond. The most common biofilter uses various sizes of gravel as the filter media in an out-of-pond box. Many include a prefilter to trap large particles before they reach the biofilter media, extending the time between cleanings of the biofilter. A bottom drain is a common feature for flushing sediment and muck out of the filter. Most filters also provide some means for aerating the water to create an oxygen-rich environment for the bacteria that colonize the media.

I decided against using gravel because I didn't want to deal with such a massive filter. It seemed to me that lighter weight materials would be easier to deal with and would not require the same strength in a container. I toyed with the idea of using cone-bottom cylindrical tanks to hold the media, since it helps concentrate the muck in a smaller volume and makes cleaning more water efficient, but I never found an economical source for these tanks. The best source I found wanted $178 for one small cone-bottom tank, and that was direct from the manufacturer.
 

I ended up buying two 100-gallon Rubbermaid stock watering tanks from a local feed store, Half Moon Bay Feed & Fuel. They cost me $94 per tank. I've since learned that they are available for substantially less in other parts of the country. I arranged the tanks side by side, with the second tank about 8" lower than the first to allow the second tank to be gravity fed from the top of the first. The return from the filter to the pond would rely on gravity also, and I therefore chose to use 4" ABS to plumb the connection between the two tanks and the return to the pond. I wanted a large pipe to ensure that I could acheive an adequate flow rate.

Relying on gravity also required that the tanks themselves be placed above the level of the return to the pond. I built two simple raised supports from concrete blocks and redwood 2"x6" lumber.

	For media, I chose medium-coarse fiber mats, available from Aquatic Eco-Systems. I cut the 24" wide, 40' roll of fiber mat into 8 pads, each trimmed to fit snugly into the tank.
	I purchased eggcrate lighting material from a local home center to serve as a shelf to support the fiber mats above the tank bottom.

	For aeration, I decided to try an aeration tower, since it seemed simple to build and compatible with my plans for an upflow filter. The 2" flexible PVC supply line fits loosely into the 4" ABS aeration tower, allowing air to mix with the water as it splashes down.
	I drilled many 1/4" holes near the bottom to allow the water to enter the first tank. I attached the tower to the tank using an ABS flange and silicone adhesive. To the right of this picture you can see the tank's bottom drain opening.
	To get the water from the top of the first tank to the bottom of the second, I used 4" ABS pipe attached to the side of the first tank, running horizontally over the side of the second tank, and finally dropping vertically to within an inch of the bottom of the second tank.

	I again used an ABS flange to attach the pipe to the side of the tank.
	Unlike the aeration tower, I used stainless steel bolts in addition to the silicone adhesive to attach the pipe, and had to cut through the side of the tank with a jigsaw. On the inside of the tank, I used both rubber and stainless steel washers on each bolt to prevent leaks.

	I was then ready to install the shelves in both tanks. Both shelves required a hole to accept the 4" supply pipe.
	The fiber mats were installed on the shelves. I'm currently using 4, 1" thick mats in each tank as my only media.
	To allow for cleaning the filter, I installed a ball valve on each bottom drain, and connected the two drain lines with 1" flexible PVC. With the filter finished, my next decision was what pump to use. I had a 1/3 HP Teel circulating pump installed in my pump house already, but it wasn't the most efficient pump and the filter would require that I run the pump continuously, so efficiency was important.
	I decided to replace the Teel pump with a Sequence 1000 1/6 HP pump from Multi-Duti. This pump had higher flow rates than the 1/3 HP Teel while using about half the electricity. It is also considerably quieter than the Teel pump in part because it pumps at half the RPMs. Here you can see the new pump installed in the pump house, with a new 2" PVC pipe heading toward the biofilter. To retrofit my plumbing for the new pump, I used flexible PVC between the basket strainer and the pump inlet. I installed a ball valve on the 2" pipe to allow me to control the flow of water to the filter.

Finally, I had to decide how to return the filtered water to the pond. I could run the return line directly into the pond, run it into the pool at the top of the stream, or any number of other possibilities. I had wanted to add a waterfall to my pond, so I decided that this would be the perfect opportunity: I would build a waterfall at the top of the stream and supply it with water from the filter.
 

With the plumbing changes, my new plumbing system was a bit more complex than the original..

Here's a picture of the completed filter. Don't worry if you can't find it, that's the idea afterall. You can just make out the brown 2" flexible PVC pipe rising from behind the short fence to the left of the garden gate. The filter itself is behind the short fence near where it meets the back fence.

As I write this, the biofilter has been in operation for about two months. After about one week, the water in the pond became dramatically clearer. I can now see the bottom, three feet down, almost as if the pond were empty. Prior to building the filter, I could barely make out the pond's white bottom drain on a good day, and not see it at all on many days. The filter needed cleaning after the first week, but went 6 weeks before the next cleaning.

I'm very satisfied with the filter so far, but will probably make a variety of subtle changes in the future, more for the fun of tinkering than for the need to improve it.

From here, you can move on to any of the other pond pages: