How to Build a Bog Filter for Your Pond

A pond filled with aquatic plants and pretty koi can be a stunning addition to your landscape. However, many people find themselves battling algae, and an overgrowth of algae is very unattractive to see. What’s worse, it can be dangerous to any water plants or koi you have in the pond, so a bog filter is essential. To keep the algae growth to a minimum, you can use a mechanical or bog filter to reduce the organic matter present in your pond and starve the algae. A bog acts like both a biological and mechanical filter.

When you build a bog filter for a koi pond, your bog area should be 10% to 15% of the surface area of the water garden, or 25% to 30% of the area of your koi pond for it to be as effective as possible. You should feed it by a pump that will move the entire volume of water in your pond in an hour or two, and we’re going to outline exactly how you go about making one below.

1 Pond
A bog filter is a natural and low-maintenance way to get a clear and healthy pond without the aid of any chemicals or a lot of equipment.

Defining a Bog Filter

A bog filter uses the natural cycle your pond ecosystem has to maintain a balance, and this improves and keeps your water quality high. Ponds that are outfitted with bog filters rarely have algae bloom issues, and when you set it up correctly, you’ll get a very good filtration system that doesn’t require a lot of maintenance. Mother nature will do all of the heavy work for you.

How a Bog Filter Works

Even though it seems far too good to be true that you’ll get stunning plants and superior water quality by using a bog filter, it’s real. It helps you create a nicely balanced ecosystem throughout your pond.

The layout for this system is nothing more than a gridwork of PVC pipes that deliver nutrient-rich water to a gravel bed in an even fashion. The bacteria will colonize in the gravel bed and help nitrify the organic waste or nutrients in the water, and this turns it into plant food. Then, these nutrients get absorbed and used by any plants you have in your pond. What you have left is crystal-clear water that is stripped of nutrients and returned to the pond.

The effectiveness of your bog filter is unmatched. You won’t need any special equipment or chemicals to use it, and it’s very low-maintenance. One of the two major issues that plague every pond owner is algae, and it can come as free floating algae or string algae. Both will take over your pond if you don’t address it, and you typically do so by installing a UV light. However, a bog filter eliminates the need for this.

Algae needs three things to thrive, including nutrients, sunlight, and water. If one of these things are missing, it won’t survive. The bog filter takes the nutrients out of the water before it brings it back into the pond. What’s left is chemical-free, crystal-clear water.

Sizing Your Bog Filter

The single most important thing to do is get the correct size when you’re setting up your bog filter system. You do this based on the size of your pond. Ideally, your bog should be 15% to 30% of the surface area of the pond you want it to filter.

So, let’s say that we need a bog filter to be 20% of the surface area of the pond to fit your needs. If you have a pond that is 15 feet long and 10 feet wide, it has a surface area of 150 square feet. The pond will need a bog filter that is 30 square feet, measuring out to 6 feet by 5 feet by 12 inches deep. Oversizing your bog is okay if you have room for it and this can only help accommodate any future needs, and it can be more forgiving if you overstock your pond.

Fish Ponds

The size of the bog filter you’ll need for a koi pond should fall between 10% and 15% of the volume or area of the pond. For a small container-based pond, the volume measurement works best as volume is very easy to calculate in smaller ponds. To make the example simple, say you created a pond using half of a wine barrel that holds roughly 26 gallons (100 liters) of water.  You’d buy a 5 gallon (20 liter) container to create your bog filter, and this is slightly oversized if you look at the numbers. However, you won’t fill it completely to the top. So, once you finish, it’ll be right around the 15% mark.

For irregular shaped, larger ponds, using the surface area to calculate the size of your bog filter is easier. Again, you’re aiming to get 10% to 15% of the surface area with your bog filter.

Swim Pond

For a recreation or swim pond that is obviously larger, the bog filter should be 25% and higher. If you decide to create an up flow wetland or bog filter, then 25% is more than enough. When we mention up flow, it simply means that the water gets pushed up through the gravel, rocks, and plant roots. This is a scaled up version of what you’ll have in  your smaller ponds.

We’ve found that overall, flow bog filters are usually more efficient as they force the water up through all of the media that has the good bacteria. These bacteria are responsible for 80% of the cleaning, and the plants cover the other 20%. There are other designs you can consider for your filter system, and they will require a larger area for the actual bog, generally working out to the same size as half of the swimming area. The fact that a simple air pump powers the whole system is another bonus.

2 Pond Size
The larger your pond is, the bigger your bog filter will have to be in order for it to keep up with the demand and be effective.

Bog Filter Setup – Important Considerations

There are three things you should consider when you design and plan your bog filter design. The water flow, movement, and size are all important. If you get these three things right, you’ll get a low-maintenance setup that will last for years in your pond.

  • Movement – Moving water too quickly through your filter won’t allow the bacteria time to use up all of the nutrients. If the water flow is too slow, you’ll have issues with stagnation, and these are pockets of water where it doesn’t move.
  • Size – The size you pick for your bog filter will depend on the use, but it never hurts to make it bigger than you strictly need. If you want to create a water garden with only a few goldfish and plants, you can easily get away with a system that is 10% to 15% the size of the pond.
  • Water Flow – The ideal water flow rate for your system is once an hour. If you have a 1,500 gallon pond, the ideal flow rate through your system will require a pump that can move 1,500 gallons per hour.

Also, if you decide to stock koi in your pond, you’ll need to increase your bog filter size up to 30% the size of the pond so it can keep up.

Bog Filter Design

Now, we’re going to dig in and tell you how to build one of these low-maintenance but powerful bog filters. The first thing you want to do is get a design in place as this can vary from person to person. You want to map out where the system will go and then excavate it to a depth of a foot. The goal is to have a flat and even surface to lay down your PVC pipes in your chosen pattern when you finish.

You’ll also have to plot out where you’re going to put the water distribution channel below your bog filter, and you’ll use 1 ½-inch to 2-inch diameter PVC piping. To figure out which size pipe is going to be best for this project, look at your pump’s output size and get the next bigger PVC pipe for the system. So, if you find that you have a one-inch outlet, you’ll want to get a 1 ½-inch PVC pipe to ensure that you get the proper flow through your whole system.

Also, instead of perforating the PVC pipe like many people choose to do, we decided to cut ⅛-inch wide slots into the PVC pipe with a circular saw that we set at ¾-inch depth. This will give the water a uniform outlet throughout the length of your PVC pipe. Your cuts should be spaced roughly three inches apart for the length of pipe.

Each section of pipe you get will cover a two or three foot width. For example, if you have a bog that is six feet wide and 10 feet long, you’ll need two lengths of PVC to run the whole 10 foot length. You want the legs of your filter system to all tie together, and you should set up your design so you have a vertical pipe at the opposite end of the inlet. The vertical pipe has to come slightly above the bog’s surface as this is what will allow you to flush the system if you need to. This is known as a clean out, and we’ll cover it more in-depth below.

How Deep Should A Bog Filter Be

Ideally, your bog filter will be no deeper than 12 inches, and it’s best if you can build the bog and pond using the same piece of liner to reduce the chances of it leaking. Any deeper than this is unnecessary and a waste.

Effectiveness Of A Bog Filter

Your pond is constantly trying to reach a state of equilibrium. Just like water tends to level out, the same holds true for any pond. Since this is a manmade ecosystem and not one that occurs naturally, you must take steps to help your pond reach this state of balance. The point you want to get to it whatever goes in has equal parts coming out, and a bog filter can help.

How to Build an Active Gravel Bog Filter – Five Steps

In nature, wetlands or bogs are low-lying areas in the terrain where fauna and flora thrive in a moist climate. Along with providing a habitat for moisture-loving organisms, it works as a filter for the surrounding ecosystem. Using this model, you can install filters into your ornamental water garden to give them a very natural appearance that is very effective. The following six steps will help you accomplish this.

1. Figure Out the Size and Excavate

If you already have a pond in place and you want to add a bog filter, you’ll excavate the spot for it right next to the pond and make it a foot deep. If you’re adding a pond and a bog filter at the same time, dig out the pond and then the space for the filter system.

Follow your usual instructions for how to build a liner pond, but you want to ensure that you leave room for your bog filter. Size the bog based on your pond’s surface area. If you just have a water garden with a few fish and plants, you’ll aim for 10% to 15% of the surface area. If you want to have a pure koi pond or keep a lot of fish, increase this size to 25% to 30% of the surface area of the pond.

3 Pond Excavation
You want to carefully excavate the area for your pond and bog filter or bog filter to ensure it’s level and not too deep.

2. Build a Raised Bog and Line It

If you want to build a raised bog filter, and 80% of people do, you’ll start the process by building retaining walls around your bog area using a combination of four-inch cinder blocks and full-sized cinder blocks that you mortar together to form a 12-inch deep pit for your filter. You can use a narrow piece of flagstone to help ensure you have spillways in place for the water to spill over and go back into the pond.

There should also be a space on the back side or side of the bog for your piping to go over the wall and down into your bog. Use a grinder to bevel the top inside edges of your cinder blocks or you can cover the edges with underlayment to soften them. Then, you’ll line your bog with a 45 mil EPDM pond liner while allowing for overlap on the top of the walls.

3. Pump Installation

Install the pump on the opposite side of your pond from where you’ll put the bog filter. This will help get good circulation of water all over your pond without any dead spots. Pick a pump that will turn the volume of your pond over every hour or two, and then run a flexible tubing along the bottom of the pond and up and out of your pond. Next, run it along and over the bog wall, connecting it with the PVC piping using a hose barb fitting that you thread into a female PVC adaptor.

4. Cut in and Set up the Distribution Pipe

Next, you’ll cut slots into your distribution pipe. The outlet of your pump will dictate your PVC size. Always bump the sizes up to ensure it uses the pump efficiently. For example, you’ll use a one-inch PVC pipe if your pump has a ¾-inch outlet. The minimum pipe size is an inch in diameter for small bogs, but we recommend that you upgrade to 1 ½ to 2-inch piping to help avoid clogs. You’ll cut slots into the pipe a third of the way into the pipe at roughly an inch apart.

Attach your vertical capped stand pipe to the distribution pipe under the gravel. Cut this pipe (clean out pipe) so it discreetly rises above the gravel bed. Spray paint the cap brown or black if you want it to blend in better.

Next, lay your distribution pipe on top of your pond liner in the area you partitioned off for your bog filter. The pots should point up into your gravel bed. Gravel bogs that are two to three feet wide can use a single pipe to feed them, but wider areas require additional lines that you space two to three inches apart. This layout is very similar to setting up a septic drain field. Make sure that each distribution line in bigger bogs has its own clean out pipes. Once you’re satisfied with the layout of your PVC piping, glue everything together and switch the pump on to see if your water is evenly distributed.

Next lay the distribution pipe on top of the pond liner in the area partitioned off for the bog filter. Be sure to point the slots up into the gravel bed.    Gravel bogs that are 2-3 feet in width can be fed by a single line of pipe.  Wider areas require additional lines spaced 2’-3’ apart.  This layout is similar to setting up a septic drain field. Be sure that each distribution line in larger bogs has its own clean out pipe. Once you are satisfied with your piping layout and location of the clean out pipe(s), glue all parts together. Turn on the pump and see if water is evenly distributed.

Why You Add the Clean Out

Don’t neglect adding this part onto your water distribution pipes. The clean out is the other end of the PVC leg that ties everything together. You have to cap it off in order for the water to flow properly, and you can do so by using a threaded plug to plug your clean out to ensure it stays in place.

Your clean out should go just above your bog’s surface. The pipe can easily get hidden with some black or dark brown spray paint. Any leg that doesn’t connect to the clean out should be capped off too. Water always follows the path of least resistance, any any pipe that you leave uncapped will release the water on the end instead of pushing it through the slots you cut in the pipe.

6. Finish the Look

Flagstone, mortar rocks, bricks, or whatever you want on the outside and top of your bog filter retaining walls will help finish the look. Shovel ⅜-inch pea gravel into your bog filter area, making sure you only fill it halfway. You’ll add the rest of the gravel when you plant. Most of the gravel isn’t very clean, so you should wash it as best you can before you add it to the bog filter. It will muddy up your pond, but you shouldn’t worry as it’ll clear up as it filters through. Once you finish with the construction process, it’s time to plant your bog.

4 Clear Koi Pond
Once you get your bog filter going, it should clear up the water so you end up with a pretty, crystal-clear pond all year-round.

Bog Filter Mistakes to Avoid

There are a few key mistakes you want to avoid if at all possible when you create your bog filter so you don’t have massive issues down the road.

  • Not capping the pipes as water tends to flow through the path of least resistance. So, it’ll shoot out of the ends instead of being pushed through the slots.
  • Not removing the plants from their pots as this severely limits their ability to absorb the nutrients and it defeats the purpose of putting in a gravel bog filter.
  • Starving the bog, and this happens when you place a pre-filter on your pump’s intake. This stresses your pump system out and it defeats the whole purpose of your bog by starving your plants of any nutrients that the pre-filter is catching.
  • Wash the soil from the roots before you plant them in the gravel. There isn’t enough nutrition in your new bog to sustain your plants, so you only want to knock the pot off the plant and put it in the soil with the roots and all right in the gravel.
  • You don’t have enough plants. You want to have one plant per square foot initially.
  • You have a gravel bed that is too deep, and this is the most common mistake that people make. You’ll need no more than 12-inches of substrate. If you’re adding a gravel bog filter to an existing area, you’ll need to use grating to build a false bottom.
  • You picked out the wrong plants for your bog. There are several aggressive species that can easily clog the pipes and grow out of your filter.
  • You picked out the wrong size gravel as you need ⅜-inch pea gravel and nothing else.

Planting the Bog Filter

Start the process by picking out your bog plants and arranging them in your bog area that you fill halfway with gravel. Make sure to stay away from more aggressive plant species. It’s also a good idea to pick out tall plants and put them toward the back of the bog filter while putting lower growing plants in front. Create interest by having contrasting plants with different textures or colors.

Once you arrange the plants how you like them, knock the potts off the plants and put the plant with the root ball intact, into the soil. Don’t remove the soil from the plants because there isn’t enough nutrients to keep them healthy in the new bog yet.

After you place your plants, you’ll gently shovel in the remaining gravel. The goal is to put the plants at the appropriate level so that when you add the rest of the gravel in, the gravel level won’t be higher than your water level. So, you want no standing water in the gravel filter area. Finish by turning on the pump and allow it to run and clear up your water.

Suggested Plants

  • Arrowhead
  • Assorted Taros
  • Blue Carex
  • Blue Moneywort
  • Blue Rush
  • Bog Lily
  • Canna
  • Chinese Water Chestnut
  • Corkscrew Rush
  • Creeping Jenny
  • Dwarf Cattail
  • Dwarf Golden Sweetflag
  • Dwarf Horsetail
  • Dwarf Papyrus
  • Dwarf Red Spiderlily
  • Dwarf Variegated Sweetflag
  • Fuzzy Bacopa
  • Japanese Iris
  • Lemon Bacopa
  • Lizard’s Tail
  • Louisiana Iris
  • Melon Sword
  • Pickerel Rush
  • Rain Lilies
  • Red Stemmed Sagittaria
  • Ribbon Grass
  • Ruby Creeper
  • Ruby Eye Arrowhead
  • Sensitive Plant
  • Siberian Iris
  • Spider Lily
  • Star Grass
  • Variegated Spider Lily
  • Variegated Water Celery
  • Water Purslane

5 Flowers to Consider in a Bog Filter
Some plants are much better suited to plant on your bog filter than others as they won’t take over or grow into the filter system.

Plants to Avoid:

Think before you attempt to add the following plants to your bog filter area. Some of them are very out of scale and large while others grow very aggressively and can get into your pipe system to block it.

  • Aquatic Mint
  • Chameleon Plant
  • Chocolate Mint
  • Gold Rush Reed
  • Horsetail
  • Mediterranean Reed
  • Mexican Papyrus
  • Native Cattails
  • Parrot’s Feather
  • Pennywort
  • Red Stemmed Thalia
  • Thalia
  • Umbrella Palm
  • Yellow Iris

Bottom Line

So, now you know how to create a bog filter. This is a very simple system that will get rid of the need to add chemicals and lots of equipment to your pond. It is a great way to get crystal clear water all year-round, and your pond will transform into a well-balanced ecosystem that will give you a spot to sit and relax for years to come.

Bog Filter 1 Bog Filter 2