How To: Build A Bog Filter


A Gorgeous, All-Natural filter for you pond!

Bog filtration is an excellent alternative to man-made pressure filters, providing both mechanical and biological filtration. While Bog Filters require significantly more space than Pressure Filters, their benefits far outweigh their spatial requirements! If properly built and designed, these filters are very low-maintenance and provide a gorgeous visual!

Below is a basic step-by-step guide to creating a healthy and functional Bog Filter; come in and ask us questions! The success of Bog Filters depend entirely upon their design; let us help you do it right the first time!

1) Calculate your pond’s size:

Calculate the square footage of your pond’s surface area by multiplying the length times the width of your pond (if your pond is not a perfect rectangle, you may need to divide your calculations into several smaller rectangles). For circular ponds, multiply 3.14 by the radius squared. The resulting number is the square footage of your pond.

In order to calculate the gallon capacity of your pond, multiply the square footage times the depth, and multiply the resulting number by 7.5. This will be important when determining flow rates!


2) Plan your construction:

We recommend that all Bog Filters are sized at 30% of the pond’s square footage, thus, a 100 square foot pond would require a 30 square foot bog. Whether this is a 5′ x 6′ bog, or a 10′ x 3′ bog, the result is the same; 30 square feet. This flexibility enables you to design a structure that fits your environment.

The Bog itself needs to be approximately 12″ – 14″ deep. This depth will allow a sufficient quantity of gravel for plant health, but is not deep enough to allow for waste buildup beneath the gravel. Bear in mind throughout your construction that you will have water flowing over one side of the bog, and plan accordingly. Though you do not need a large gradient to allow water flow, the structure on the pond-side of the Bog is generally lower than the rest of the Bog in order to allow easy flow into the pond.


3) Construction:

It is now time to build the walls (or other framework) around the Bog. It is important to remember that the Bog Filter must sit higher than the pond; without this, water will not properly flow into the pond. If you choose to use liner (and we recommend it), consider adding underlay beneath the liner to protect it from rocks or other sharp edges. With or without the underlay, ensure that the walls of the structure are built to hold the liner in place. There are many ways to do this, so if you’re unsure of your methods, come in and see us!


4) Liner and Piping:

Place your liner into the structure. Ensure that the liner is properly restrained and protected; a damaged liner will not hold water! Once the liner is properly inserted, begin filling the Bog with water.

Once the water fills 80% of the Bog, it is now time to add your output piping. We use machined, slotted piping to evenly disperse the water through the gravel. This piping is precision-cut for this purpose, and it performs beautifully. It is absolutely critical that this piping be properly sized with your pump and the square footage of your bog; if done incorrectly, the Bog will develop stagnant areas, resulting in annual cleanings, requiring all of the gravel to be removed by hand. Because this step is so critical, we encourage everyone to come out and discuss this step with us before proceeding; we can make sure you do it right!


5) Add Gravel:

Fill your Bog with 10″ – 12″ of 3/8″ pea gravel. Make sure that the gravel is pre-washed or, if it is unwashed, wash it yourself; the dust of the gravel will wreak havoc on water clarity! When adding the gravel, be careful that you don’t damage or shift your output piping; the pipes must stay in their designated positions!

Though we highly recommend 3/8″ pea gravel, anything in that general size will work. If the gravel is too large, the plants will not be rooted as well, and if it’s too small the water will not flow properly. We recommend Keller Materials or Brookstone Ranch for your rock needs!


6) Test Run:

Turn on your pump and run the system. Pay close attention to your elevations and flow; there should be between 1″ and 2″ of water above the gravel. If the water flows properly and evenly throughout the gravel (no geysers and no dead zones), you have successfully constructed a Bog Filter!

Throughout this phase, minor modifications can be made to your water flow and gravel levels to better suit the desired visual or re-level the Bog Filter.


7) Add Plants:

Once your Bog has successfully run water for several days, you may add plants. If this is a new pond, it is important to remember that the water must be dechlorinated before the plants are introduced. You can introduce plants the same day that the water is conditioned; let us know you need a dechlorinator!

Choosing your plants wisely; we carry over 50 varieties of Bog Plants that are perfect for Bog Filters! Some crawl across the surface and spread rapidly, while others stay localized and grow tall. Though there are no bad choices for Bog Filters, some plants do filter the water more efficiently than others, and a member of our staff will be happy to help you select the best plants for your specific environment.


8) Maintenance:

Your plants will need to be trimmed back annually, and occasionally thinned out. Full cleanings may be required occasionally, but the frequency of this is largely determined by fish load and Bog size, so you will need to decide for yourself when the gravel needs to be cleaned.

Whether a full clean out is needed every other year or every ten years, this filter is one of the lowest maintenance systems available and, perhaps best of all, all natural!



Bring us your design and we’d be happy to consult with you! We’ve helped design many Bog Filters (and built several ourselves); ask us questions!

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