One of the most important aspects of maintaining a healthy, attractive backyard pond is filtration. Fish and decomposing plant material are constantly adding nutrients and chemical waste products into the water. If these build up in the water, it can cause problems for the fish and will contribute to the algae problem. Filters help bring the pond back into natural balance which will tend to keep the fish, plants and you happy.

About the only exception to the rule is a small pond with only plants or one or two fish. Often such a small volume of water can be kept in a natural balance without external filtration. However, for most backyard ponds, some way of removing excessive nutrients and solids from the water. 

Physical Filters - The idea behind a physical filter is to trap particles as they move through the pond system. Screens and layers of filter fabric material provide a level of filtration for the larger particles in the water. They may become clogged over time so they need periodic cleaning to remain effective. Skimmers are often used in a filtration system to catch the larger items such as leaves from nearby trees or, heaven forbid, dead fish.

Biological Filters - Biological filters use nature to remove nutrients and harmful byproducts from the water in the pond. Having success with a biological filter is similar to having success with a compost pile. In both cases, the goal is to provide the proper conditions so that bacteria can survive and multiply. It is these microscopic critters that eat the nutrients and byproducts and turn them into more inert substances. When they take up the extra nutrients in the water, they deprive the algae of needed fertilization. 

For the bacteria to thrive, they need some type of substrate to anchor onto while the water flows past. This may range from sand to nylon bags filled with volcanic stones. Whatever the substrate, it needs periodic cleaning but you do not want to bother it more than once a year or so. Each time the substrate is flushed with a blast of clean water, the bacteria will also be removed. Once they are placed back into the pond system, it will take some time for the population of bacteria to rebuild itself. Thus, it will also take time for the filter system to become fully functioning again. 

Bottom Filters - Ponds that have small stones covering the bottom present other options and problems. In an aquarium, bottom filters are used to draw waste products down in the stones where they are acted upon by the bacteria. The problem with using a similar system in a backyard pond is that they are much more difficult to clean out. The pores between the stones become clogged requiring the removal of all the stones to allow for adequate cleaning. This is a lot of work!

Ultraviolet Filters - Another approach to algae control in the backyard pond is to expose the algae to ultraviolet light. This is supposed to kill the individual algae plants to keep the populations low.

The water is piped through a unit similar to the one displayed above. It is connected to an electrical outlet for power to run the ultraviolet lights within the tubes. As the algae pass by the light, they are killed. 

Big Time Koi Ponds

If your goal is to raise champion quality koi, your filtration demands will generally be greater than for the average backyard pond. Often, you will be growing a larger number of fish of larger sizes that produce greater amounts of waste. 

To keep the ponds clear and healthy, will also require more elaborate filters that may require more frequent maintenance. Large filters located outside the pond may be used to filter out extremely fine particles and to remove high levels of nutrients and mineral waste products. 

If this is your situation, you may want to check with the Koi societies and organizations for advice on recommended filtration systems. 

• Biofilters are often combined with the origin of a waterfall for your pond. The water is pumped into the bottom of the waterfall and it moves upward through layers of foam and then into a media where the bacteria live. It then discharges over the front side creating the waterfall.

• When installing the biofilter, be sure that the top is level before filling in around it. If it is not level, water will flow in an irregular pattern and give a "lopsided" appearance to your waterfall. 

• Bacteria need to have something to live on as the water flows over them. Porous material such as lava rocks provide a lot of surface area and are light weight for their volume. 

• A nylon mesh bag makes a nice container for the biofilter media. It makes it easier to remove the media for cleaning and allows thorough movement of water in and among the bacteria laden stones.