• Once upon a time, finding a filter for an outdoor pond was simple; you divided the volume of the pond by half to find the size of pump needed to power a canister filter. Today, a 1,200 gallon pond is considered a modest home for half a dozen foot-long koi and the choice of filters depends on construction, waterfalls and other components that we have come to expect in our "water features".

    Plants, Fish and GPMs

    A "natural" 1,200 gallon pond with three-quarters of its area filled with plants wouldn't need a filter but if you want fish in that pond, that many plants will interfere with your enjoyment of the fish. If you plan to keep koi or goldfish---both members of the carp family---you will definitely need a filter to keep up with the waste produced by these heavy feeders. You have room for 12 pounds of fish in a 1,200 gallon pond; six healthy koi about 10 inches long (about 6 pounds of fish) will have plenty of space at the beginning of the season but you could have almost three times that weight if all of your fish survive to the end of the season. Plant more green and start with fewer fish---and buy a bigger filter pump than you need. To move a minimum of 1,200 gallons in two hours (the recommended rate), you'll need a pump that moves at least 6 gallons per minute (gpm) for a simple garden pond. In a 1,200 gallon koi pond, the flow rate should be greater---13.3 gpm or more. To figure volumes and flow rates, use calculation tools like the ones provided in "The Complete Pond Solver", by Tony Roocroft and on the web site, World of

    Maximum Lift

    The next factor in choosing a pump is to find one capable of "lifting" water from the top (or "head") of the pump to the outlet at the top of the filter. This "maximum lift" should be included in the circulation rate of the pump. Check filter pump package "specs" for elevation and accessory "max. lift". Fountains are attractive and fish like moving water; a fountain doesn't create a current but a waterfall on the end of the pool will develop a flow at its base and around the sides of the pond. Pumps that allow for elevation and accessories and provide at least an 800 gallon per hour circulating volume (13.3 gpm) will have enough power for a 1,200 gallon pond.

    Biofilter Basics

    "Biological" filters are much better at sustaining pond ecosystems than simple mechanical systems that strain solid matter and use chemicals to remove nitrites. Biofilters sit above the pond and flow back into the pond by way of gravity using a waterfall or return pipes above the pond. After the solids are removed, water moves through a chamber where bacteria change ammonia to nitrates. Tony Roocroft, reports that "one-third of the pond size" is a good measure for a filter but that the filter's efficiency is more important. Under the one-third guide, a biofilter for a 1,200-gallon pond that was, say 10 feet long, 5.5 feet wide and an average of 3 feet deep would require an area of about 53 cubic feet unless the filter was very efficient. Aquatic supply professionals or an experienced local koi or garden pond member (See Resource 2) can help interpret ratings for specific applications.


    Practical Water Garden Solutions: How to Select a Fish Pond Filter

    Super Pumps: How to Specify Any Submersible Pond Pump

    Super Pumps: Calculators


    Olympic Koi, Golfish and Garden Pond Clob: Pond and Filter Design Issues

    World of Water: Guide to Filtration

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