A bog sounds pretty ominous: a dark, dank area where creatures of legend are poised to do things not in the best interests of a heroine or unsuspecting law enforcement agent.

But really all a bog is, according to the handy dandy dictionary, is “wet muddy ground too soft to support a heavy body.”

What is a Bog Filter?

In its essence, a bog filter is a biological filter, whose primary purpose is to add filtration to a pond.

A bog filter is an area—either in or near the main pond—that is covered in a substrate (usually pea gravel) in which aquatic and semi-aquatic plants live, and through which the pond water is pumped (ordinarily via a perforated manifold system of PVC piping).

Ideally, the bog filter will grow beneficial bacteria in a more natural setting, separate from the pond and other methods of biological or mechanical filtration. This bacteria helps remove ammonia and other harmful nitrogenous waste products during the nitrogen cycle in order to feed the aquatic plants that are situated in the bog filter. In doing so, the bog provides clear and clean water to the koi pond.

⅜” pea gravel is usually the recommended substrate because

  • It allows enough surface area for beneficial bacteria
  • It is more conducive to root/plant growth
  • It doesn’t clog as easily as smaller-sized gravel
  • It won’t restrict or divert water flow as badly as larger-sized gravel

How Does It Work

The water from the pond is pumped into the bog through perforated PVC pipes. This waste-filled water from the main pond moves through the substrate, and the aerobic, nitrifying bacteria on the substrate goes to work on the nitrogenous waste, breaking it down into nitrates that plants can use as part of the nitrogen cycle.

Once the waste is in its more nutrient-rich form, it creates a microscopic organic smorgasbord for plants. The plants, the final component of the bog’s filtration line, then scrub those nitrates from the pond’s water column before the cleaned water is reintroduced back into the main pond. The bog is filled with various aquatic plants which feed on the byproducts created during the breakdown of nitrogenous fish wastes and other organics.

If you’re looking to install a bog filter, the generally accepted size ratio for effective break down of the expected waste load is 25-30% of the koi pond’s surface area. That ratio drops to 10-15% if your bog is only filtering a water garden.

Nitrogen Cycle in a koi pond




It is important to remember that all filtration has to be commensurate with the amount of waste that is generated. If you have a big fish load, you will have a lot of waste and will likely need a bigger filtration system.

Your pump should circulate the entire pond volume in 2 hours. Ensure that your pump is big enough to handle the gallons per hour needed. Here’s a handy pump size calculator from the Pond Informer

A koi pond is a closed system. The same water recirculates over and over. Freshwater is only introduced via water changes, not a normal flow system (as it is in nature). A bog system works best with water in an open system where the water moves on to be replaced by fresh water.

Maintaining a Bog Filter

Unsurprisingly, this method of biological filtration draws a lot of support as well as some pretty serious criticism from koi and pond aficionados.

The benefit of bog filtration that is almost always mentioned is that it is a relatively low maintenance method of filtering out waste from the water column.

That said, many people underestimate the maintenance required for this type of filtration.

Despite your bog’s ability to break down waste, mulm and sludge will eventually deposit on the pea gravel as fine particulates and other solid wastes begin to accumulate. Getting rid of this build up should be done at least twice a year. If neglected, you will be left with an almighty deposit of foul smelling gunk on the bottom of your bog. This, as many pondkeepers will tell you, is a tremendously tedious and stinky job.


CAUTION: Over time, if not monitored, perforated pipe can, and will, clog with debris if it is not removed from the water.

Bottom line is that there is maintenance required on all types of koi pond filtration. Some of it is less time intensive and requires less elbow grease, but some cleaning and general upkeep are required on all koi pond filters.

Types of Bog Filters

At its most basic, a bog is simply a depression wherein water can settle and plants can grow. There are a few different constructions of filters used by enthusiasts.

Raised Bog

As the name suggests, the bog is constructed at a higher level than the pond, and the water trickles back into the pond via waterfall or stream.

Although not in the true spirit (or actual methodology) of a bog filter, some enthusiasts will raise the gravel (or substrate) above the bottom of the bog. This allows some of the mulm to fall through. This “muck” is then captured in a settlement area (similar to a traditional mechanical filter) where it is removed—or flushed—through a bottom drain.

Partition Bog

Again, as the name suggests, this construction is part of the pond, but separated from the main body of the pond by some form of porous material that acts as a barrier or retaining wall.

partition bog filter in a koi pond

Island Bog

This is essentially a partition bog but in the middle of the pond. The porous barrier encircles the bog keeping the substrate in but allowing the water to flow through.

Border Bog

This bog surrounds the pond as a shallow ledge with a porous barrier separating it from the pond, through which the filtered water can pass while retaining the substrate.

You don’t need a really deep bog filter. One foot deep is usually enough. Deeper than that can lead to low-flow areas and anaerobic zones, which can lead to an excess of toxic by-products. Here is a look from GrayStone Creations about some of the design considerations to take before building a bog filter.

Pottery Bog

This is usually used in ornamental ponds. An ornate pot or container holds the substrate and the water flows back over the sides of the pot.

Pros of a Bog Filter


The flora in bogs provide a smooth transition between pond and land. Plus, plant life is lovely to watch as it blooms.

Stunts Algae Growth

As the plants in a bog naturally remove the same stuff that the algae would ordinarily feast on, bog filters help to keep algae blooms to a minimum.

More Natural

Bogs are how Mother Nature turn waste into plant food in more pastoral settings. Pondkeepers who prefer to keep their pond as natural as possible often opt for bog filters.

Cons of a Bog Filter

Mulm Buildup

The detritus that falls to the bottom will accumulate over time and can pose not only a clean up headache but an anaerobic environment, too.


A bog is still a form of filtration and, as such, it will have to be maintained. Organics eventually accumulate, and left uncleaned, they will generate anaerobic muck that harbors parasites as well as large numbers of bacteria. Untended buildup can create hydrogen sulfide and methane gases which not only smell, but can also be fatal to your koi.

Makes Parasite Treatment Difficult

Most pond-wide parasite treatments rely on oxidizers (like potassium permanganate), which will wreak havoc on your plants. Using these treatments could potentially annihilate your bog vegetation.

Some Things When Planting Your Bog Filter

Know your region. Some plants do better in different regions. Look up your zone and see which plants work best for you.

From Dwarf Cattails and Iris to Creeping Jenny and Lettuce, here is a list of some of the types of plants your for bog filter from Nelson Water Gardens

It’s also helpful to know your plants. Some species can be extremely invasive and will take over your bog, clogging pipes and even filters.

Mix up your verdure. You can choose from tall, semi-aquatic plants whose fibrous roots are great for microbe growth (and “cementing” the substrate), or marginals that will inhabit the edges of the pond.


Wash the gravel before use. You never know what is on it and how your seedlings might respond to any chemicals picked up during the manufacturing process.

Look into growing some edible water plants. Celery and lettuce can be great additions to your bog plants.

Remember that all plants have roots. These roots over time can raise the level of the substrate. Be aware that this can impact the ability of the bog to move water back into the pond.

Go Substrate-free!

You can build a beautiful bog without substrate. Just weigh the plant roots so they’ll stay submerged. The plants don’t need substrate to thrive and use excess nutrients.

Make sure you have enough plants. One plant per square foot is a good place to start.

Take the plants out of their pots. The plant and its roots need full access to the nutrients. Leaving them in the pot severely limits that access.

Resist the urge to knock all of the potting material off your plants before planting them in your bog. If it is a new bog, the plants will need the nutrients to sustain themselves (and don’t worry, the soil won’t lessen the bog’s effectiveness).

It is important to remember that bog filtration is ONE way to help with the Nitrogen Cycle and waste removal in your koi pond. It works for some, but not for others. A healthy debate can help to fuel new ideas or different ways of thinking, and many have switched to or from one type of filtration to the another.

3 responses

  1. I’ve had bogs in all my ponds and detest cleaning time. A substrate free bog sounds like a dream, but I Googled it and found nothing. Could you possibly provide some further information? Thanks so much!

    1. Regrettably, we are not able to offer any more information on substrate bogs Holly. Sorry to hear that search results have not proven helpful. Perhaps a koi blog/community would have some first hand accounts of those with experience with substrate bogs.

  2. I took all my gravel out of my bog, because once it gets dirty, its almost impossible to clean. I know use only plants. Water celery roots work great and are easy to pull up when I clean it.

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