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Ok, so you’ve decided you need to filter your pond. The green algae is getting out of control, and your fish are gasping for air ever time you turn around! It's time for some serious help. Filters come in 4 main types: vegetable, bio-filtration, mechanical filtration, and surface skimming; this article focuses mainly on bio-filtration. Bio-filtration requires setting up a way for nitrifying bacteria to colonize your pond and help convert ammonia into nitrite and finally into nitrate. These bacteria are aerobic and require oxygenated water flowing past them or they suffocate. This type of filter should take up 10%-20% of the total volume of your pond. Several styles of bio-filtration systems exist: an up-flow system, a horizontal flow system, bubble-bead and fluid bed filters, and vortex system. Choosing the right filter is important to the size of your pond and the amount of time you want to spend maintaining it. A medium such as lava rock should be provided with a slow flow of water 1-2 gallons per minute of medium surface area. Other types of filter media may include volcanic rock, bio-balls, or bio-ribbon. If you are pumping water into this system, it is a good idea to cover the medium with a layer of plastic foam or other porous filter to keep large particles of sediment from settling on your bacteria colonies and suffocating them. An up-flow system usually uses gravity and a screen helps catch solid particles that have sunk out of the water flow before being filtered back up into the pond via the medium where they are further cleaned by the bacteria. Horizontal flows create a system that creates zigzag flows of water where heavy particles sink to the bottom of the system and can be washed out later. The water is then fed back through the bio-filter. Start a bigger pond’s filter with a bottom drain. You will pump water out of the system for the vortex based system and bubble based systems. Using a sump pump is generally less efficient that using these types out-of-pool filters. Bubble-Bead systems pump water into the bottom of a tank filled with small floating balls. About 1% of the water-volume of a pond must go through it for it to be effective. As the water is drawn up, heavy particulates stick to the balls; they are also colonized by aerobic bacteria that help further filter the water. They are usually set at some distance from the pond and can be disguised by landscaping. Fluid-bed filters use a similar concept. Both require daily flushing of the media to remove particles. Systems used on ponds over 20,000 gallons usually have a self-cleaning mechanism built in. Vortex filters involve a series of 3-4 cone shaped vats installed at some distance from the pond. Plumbing carries water to the first chamber where it forms a whirlpool and heavy particles fall out. Water exits through a side into a second vat, where varying porous mats are installed to filter out more debris. A third chamber contains bio-media that help filter out ammonia and nitrate. Water is then returned to the pond via a waterfall or other such water feature. Source: The Living Pond: Water Gardens with Fish & Other Creatures by Helen Nash
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