Build a DIY bog filter for your pond using gravel, plants, and a trough to naturally filter water and reduce algae—eco-friendly and low maintenance!
When we went to put in a pond at our new house, I did a lot of research and stumbled upon the use of bog filters as an alternative to a traditional filter. At our last house, we used a filter with UV lights which worked well, but as with all things, can fail. While this option certainly can fail as well, I liked the idea of using natural methods to improve the water quality vs. trying to do it with artificial means.
In this post, I’ll cover how we created our bog filter… this has been a learning process and I’m sure that I’ll learn more over the next few years as we see what works or doesn’t work.
What is a bog filter for a pond?
A bog filter is a natural filtration system used in ponds to help maintain clean, healthy water by using plants and beneficial bacteria to do the work of a traditional filter.
It typically consists of a shallow, gravel-filled area where pond water is pumped through before returning to the main pond.
As the water flows through the gravel, beneficial bacteria break down waste and excess nutrients, while plants absorb those nutrients, helping to reduce algae growth.
Bog filters are low-maintenance, eco-friendly, and enhance both the beauty and functionality of a pond by supporting a balanced aquatic ecosystem.
To put it simply: There is a pond pump (still need one of those) that pumps water into the bog. In our case, the water enters at the bottom of the horse trough. As the pump pushes more water in, the water rises through large rocks, then medium rocks, then pea gravel, then through the plant layer, then spills out of the opening.
The large rocks trap larger debris, the medium traps smaller debris and so on… the plants help filter the water (as plants do).
While bacteria SOUNDS bad, beneficial bacteria in the water is often helpful in clearing and cleaning water. It’s part of the natural cycle and it’s something you want, both in fish tanks and ponds.
Bog filters function as natural marshlands or wetlands, they just work on a much smaller scale. If you want to see a really amazing wetlands setup, check out the wetlands at the National Aquarium in Baltimore’s Inner Harbor. They’re absolutely stunning.
Bog Filter Size
- The bog should be about 30% of the size of your pond.
- Flow rate for the pond pump that moves water through the bog should be equivalent to the bog size x 6 (gallons through the bog per hour). You need a higher powered pump if it will also be controlled streams or waterfalls.
Our pond contains approximately 4,200 gallons of water which would indicate that we should use a 1,260 gallon bog, and that our pump should push through 7,560 gallons of water per hour. This is overestimating the amount of water as the shelves and varying depths in the pond make it difficult for me to calculate perfectly.
We used a 300 gallon stock tank meant for horses so clearly, our bog filter is undersized. We went into this project understanding that and realizing that we may need to run additional filtration potentially. We are, however, hoping that the rest of the plants in our pond may help provide additional filtration, and the large shallow shelves may function similarly.
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Please read the whole post so you don’t miss any important information!
Video of Our Early Bog Filter
This is a video from when we were setting up the bog filter, just to show you what it looks like!
DIY Bog Filter with a Stock Trough
Project Supplies
- Wool (absolutely optional)
- 300 gallon+ horse trough
- PVC pipes and connections to hook your pond pump to the bog filter
- Pea Gravel
- 1-3″ rocks
- 4-6″ rocks
- Plants that like wetlands/bogs.
Step by Step Instructions
Step 1: Rinse your gravel lightly, as well as your trough. Place the trough where you want it. I needed it on the liner or close to the liner because the water spills to the front and it needs to roll down into the pond again (versus into my garden which will very quickly result in an empty pond).
In the photo below, you can see where the pond liner was by the time I was finished…
Step 2: Place your bog filter at the opposite end of the pond from your pond pump. This helps circulate the water. You want to connect the tubing for the pond pump to the bottom of your trough so the dirty water enters at the bottom.
This is a photo of our crappy pump from last year and the ‘cage’ we used to help keep excess yuck out. It didn’t work very well… none of it did. So it got clogged frequently and was a lot of upkeep. But the tubing worked fine.
Step 3: Next, you need to set up your PVC for inside the trough. The two far ends are capped eventually, but you drill small holes inside the tube to allow water to flow up evenly into the trough.
In the last photo, you can see the PVC ready to be connected to the pond pump. The trough drain is facing the opposite direction… this is the clean out which may be needed for routine maintenance.
Step 4: We cut a hole in the front of the trough to insert the PVC through to connect everything to the pump.
In this photo, you can see the PVC tube on the exterior of the trough, entering through the front. There’s a connection to use to hook up the pump tubing.
Now the tube is hooked up to the trough below. Make sure to give this a test run before you fill everything in with rocks around the bog filter.
Step 5: Optional and probably silly… I added my layer of large rocks, then added a layer of wool. I was thinking that it might help with additional filtration. I like to experiment.
I added the 4-6″ of large rocks first, then the wool, then the 1-3″ rocks, then 12″+ of pea gravel. The height of this bog filter meant that it required a LOT of rock… I’m pretty certain it was a lot more than 12″ that I ended up needing.
I ended up using three small PVC tubes for the water to exit (otherwise it’ll overflow over the top of the trough, all around it). That eventually was expanded when we upgraded to a bigger pump. I used small baskets to keep pea gravel from exiting with the water.
Step 6: We added lots of plants at this step. There are quite a few that can work well such as swamp milkweed, iris, cattail, creeping jenny and joe pye weed. I tried strawberries, but they didn’t do very well in mine, but they might work in yours. Creeping jenny LOVES it and was affordable and fast spreading. It’s low and spreads so it’s good for the front of the bog filter while taller plants work well in the back.
Step 7: Last we added stones to the front so the water could waterfall down out of the trough. The water in the pond was GREEN and this is what it looks like coming out now.
Photos from May 2025: These are updated photos of the pond and bog filter!
One Year Later…
Resources about Bog Filters:
- Nelson & Water Gardens Nursery (best article in my opinion): https://nelsonwatergardens.com/gravel-bog-filter-construction/
- Backyard Water Gardens Website: https://backyardwatergarden.com/how-to-make-a-bog-filter/
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