Pond Tips - Home made filter materials page #2
For ponds under 300 gallons read the blog "Filters for Tiny Ponds
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There are numerous materials available reasonably that will do wonders toward cleaning your water and making it healthy for your fish.

A pond filter needs to do two jobs. The first is to remove dirt from the water.
The second is to convert ammonia (fish waste) to harmless forms.
This is a very basic description of some very complex functions. However, a small pond owner does not need to be a professional to understand some simple steps that are guaranteed to produce success.

Cleaning is a matter of separating the tiny dirt particles from the water.
Biological filtration is  providing a home for the microscopic microbes that eat ammonia and pass it off in a harmless form.
Lets talk about the dirt first. Everything that ends up in your pond eventually breaks down. In time, even large leaves will become millions of tiny specks of dirt. The dirt becomes home for algae, and soon each speck becomes a much larger floating green algae ball. This is what turns your pond green. Removal of suspended algae is best done with a very fine mesh filter material. There are numerous products available. We have found that nothing works as well as filter cloth as the final stage media.

The biological function requires a huge amount of surface area thru which the water flows. The microbes living on these surfaces feed on the ammonia as it passes. If you imagine a kitchen scrubbing pad, it has a course gridwork which has a large surface area. You need to provide a similar but much larger volume of media for your biological filter. The most popular media, and the easiest to clean is a rigid pad placed vertically within the filter box.
A large sturdy waterproof box houses numerous vertical layers of these pads.
Your ammonia laden water flows into one end, the microbes eat the ammonia, and after it is processed the water flows out the opposite end.

A combination filter that does both functions is ideal. Using several different densities of media, getting tighter as the water flows along, and a cloth final stage is an excellent example of a great pond filter.
I mentioned "vertical" pads because this design is the easiest to maintain. It takes just minutes to lift a vertical pad out and clean it with a hose. All designs with horizontal layered pads present a major amount of work to clean. The worst designs are pits full of gravel of any type. They work OK for short periods of time, then trap sediment and create a dense anerobic stinking mess. This requires total removal of the media for cleaning. It's a job that you will never want to do twice. Gravel or sand can only work on clean water like a swimming pool. Unless all sedment is removed BEFORE the water passes thru it, all filter material becomes blocked. This happens far more rapidly than most people imagine. Back washing is successful on chlorinated water. The chlorine kills all forms of life, leaving sediment that is not sticky like pond dirt. Pond sediment is alive. It contains millions of micro-organisms that are busily at work colonizing their environment. To clean pond sediment by back washing, you would have to do so often, so much so that your water bill would go thru the roof. Large installations can buy expensive equipment that scrubs the water clean, and saves the wash water in settling ponds where it is re-used. Typical pond owners do not have the resources for such elaborate equipment.

Proper pond filter design requires a material that can be cleaned easily, and lasts a very long time under the abuse of handling and hose water pressure. Once again, I am going to recommend a rigid vertical plastic pad. There are many other similar materials. No other design works as well. Each has pros and cons. Read up on what is available and let your budget help you to decide. My personal preference is Matala, the only negative is it's cost. Matala is a permanent, sturdy, rigid mesh filter material that has been handled regularly in my own pond for 20+ years and is still in great shape. Remember, if you buy and replace just once, your real cost doubles at a minimum.

So, what does it take to get the job done right? Well, lets start with the size of the pond in gallons. Then divide that number by 200. The result will give you a reasonable number of cubic feet of media material and the inside volume of the needed filter.
A 10' x 10' pond that is 30" deep = 1875/200=9.375 cu ft.

A filter box that is 36" x 20" and 24" deep = 10 cu ft, and a good match.
If the pads are 1-1/2" thick, you can get about 16-18 of them in the box and leave space for a spreader grid on the intake and a cloth filter on the exit end.

A good installation would be to place this box out of sight just behind a waterfall.
A pump would suck water from the far end of the pond, send it to the filter, then overflow it by gravity to the waterfall. In very sunny or dusty areas, an inline UV filter could help reduce algae buildup. In warm areas an air system would maintain dissolved oxygen levels and keep your fish breathing if the water climbs over 80F degrees.

It is most economical if you search for materials information before starting to gather parts. The filter box will determine pad dimensions and your ability to build this filter could make it more desireable to purchase a pre-built system. The use of concrete or block can produce a box of any size, but these are much more permanent and harder to modify later. The larger the project, the more time you should take to work out the details. A pond can be as small as a half barrel or animal watering tub or as large as your property and the county allow.
We have 45 years of experience in this field, and advice is free. I do have a set of plans for a hand built filter. These are free by eMail upon request.