How Kris Turned an Old Pond Into a Powerful Bog Filter

Hi Kev,

About two years ago, I upgraded my small 365L pond to a larger 1475L one.

My garden is sloped, so I decided to build a terraced pond to keep things simple and cost-effective. Instead of throwing away the old pond, I reused it as a bog filter.

I had zero experience with plumbing, so there was a lot of planning involved… but in the end, it all came together.

The result has been amazing — the water has never been clearer. The old UVC filter is now just a backup, and the pond has been running on the bog filter alone for over a year.

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Stocking and System Setup

The pond currently has:

• Around 15 Sarasa and Shubunkins
• A few freshwater mussels
• Two small koi

Everything is doing really well.

According to your calculator, the bog filter is roughly 25% of the pond volume, so it should be more than capable of keeping things clean — and so far, that’s proven to be true.

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Building the Bog Filter

Inside the bog filter:

• I used large rocks on the bottom layer
• Filled most of the space with clay pebbles to keep costs down
• Finished with a top layer of decorative pebbles

There’s about 8cm of empty space at the bottom, created using plastic grid tiles to hold the rock layers up.

The intake pipe directs water at a 90° angle into a large rock, which helps spread the flow evenly across the filter.

I also added:

• A mesh bag with charcoal filter material for easy yearly replacement
• A shallow edge where birds can drink and bathe

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Plumbing, Flow, and Tweaks

The system went through a few iterations.

• I initially tried a more complex outflow design, but ended up switching to a simpler pipe with vertical cuts and holes, which worked much better
• The breather pipe is slightly submerged to reduce noise
• I attempted a venturi setup for extra oxygen, but that didn’t work — so I now use an aerator pump instead

I also added:

• A valve on the intake pipe to isolate the pump for cleaning
• A water level sensor that shuts off the pump if levels drop (in case of a leak)

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Pipe Sizing (What I’d Change)

I used:

• 32mm intake pipes
• 42mm outflow pipes

In hindsight, I would have gone with larger outflow pipes, but I managed to make it work.

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Climate Considerations

I’m based in Belgium, where winters can drop below freezing (usually not below -5°C).

To handle that:

• I added insulation around the bog filter tub
• Insulated the sides of the pond
• Insulated the inlet and outlet pipes

If needed, I can also add a heater during colder periods.

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Results

The plants are growing like crazy, and the water is consistently crystal clear.

Nitrite and nitrate levels are always at the lowest reading on the test strips — essentially zero.

I do get some string algae in early spring, but it clears up after a few weeks once the plants really start growing.

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Final Thoughts

This whole project started after I came across your channel.

It really inspired me to give it a go, and I’m glad I did.

I didn’t spend a fortune, I learned a lot along the way, and it’s been a really enjoyable build from start to finish.

Thanks for the guides — and keep the content coming!

Kris

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A Word from Kev

Every pond I build follows the same basic principles…
Kris didn’t copy my pond — just applied the system in a way that worked for their space, climate, and what they had available.

That’s really what this is all about.

You don’t need perfect materials or a perfect plan — you just need to understand how the system works, and then adapt it to your situation.

I also like how Kris reused the old pond as a bog filter. That’s exactly the kind of thinking that keeps costs down and still gets great results.

You can see it in the outcome — clear water, healthy fish, and a system that’s been running on the bog filter alone for over a year.

Thanks to Kris for taking the time to share their build and lessons learned with the wider pond community — it genuinely helps others build with more confidence.

If you’ve built something you’re proud of, I’d love to see it.
You can share your pond here.

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