How To

How to make a DIY pond filter that will keep your Koi fish happy

By Luke Carlino

Updated: August 24th, 2021

Ponds can get dirty, learn how to rectify this easily!

If you have a pond, a pond filter is a must, and an easy way to solve this is by making your own DIY pond filter! This is especially important if you keep fish in your pond as they deserve a nice place to live.

You can always purchase a commercially available filter, but if you want a cool little DIY project and don’t mind saving a buck, build your own!

Read on for our guide to building an athletically pleasing and easy to maintain DIY pond filter and keep your pond clean and clear all year round.

Equipment needed to create a DIY pond filter

  • A container that is UV stabilised and food grade
  • PVC pipe that is slightly larger than the recommendation for the pump
  • Bulkhead fittings to make a watertight seal
  • A drill with different sized drill bits
  • A valve, plumbing elbows, caps and various fittings (will vary based on your project)
  • Different sized rocks
  • Water plants
  • Something that can create a void at the bottom of the filter like a large slotted pipe, old pots, etc.

For your food safe storage container, you should be able to find these at any hardware store. The goal is that your container holds 10% of the overall pond volume. So, a 1000L pond needs a 100L container. Multiple containers can be used if necessary.

If you have a really big pond, rather than finding a huge container or lots of containers, excavate a hole, then use a pond liner to create a reservoir that will become a filter.

Large fish means more waste and more filtration 

Roughly 30% of your pond should be a filter but if you have larger fish, aim for 50%. It sounds like a lot, but this DIY pond filter is natural and easily blends into the pond landscape.

Making sure your filter is effective requires these four things:

  1. Remove all solids
  2. Slow the water flow
  3. Provide a home for beneficial bacterias
  4. Have plants to remove nutrients
DIY pond filter media
Source: Maja R. via Unsplash

Removing solids

An effective pond filter removes solids like fish waste from the pond. The pump will suck them up and deliver them into the filter. You can then remove these every now and then by adding a tap:

  1. Drill a 25mm (1”) hole at the lowest point possible on the container
  2. Use a bulkhead fitting to provide a watertight seal
  3. Connect 25mm (1”) PVC pipe with a valve

Now you have a tap that can be opened to flush the solids out.

For larger systems, you will need a sump or cleanout pump to achieve the same action.

Slowing water flow

Slow the water flow so that it moves evenly through the pond water filter. The water must make contact with the beneficial bacteria, which performs the role of filtering.

Do this by connecting a pipe from the pump into a pipe that sits in the filter. Drill holes evenly along the pipe with an 8mm drill bit to distribute water evenly through the filter. Let the pipe sit on some rocks to avoid it sitting directly on the bottom of the container. The solids will accumulate in this void, then be removed via the tap.

When drilling the holes along the pipe, it is important to keep the spacing nice and even. the goal is to ensure that the holes are large enough for the solids to pass through. Make sure the pipe is level, so it distributes the water evenly.

Creating a home for bacteria 

Beneficial bacteria is a very effective water purifier there, so the more of this, the better. This bacteria will break down the ammonia and nitrite into just nitrate. These are toxic to fish and can be a food source for algae.

Bacteria require surfaces to grow on, and the rocks and gravel in the filter will provide this.

Before you fill the filter with rocks and gravel, ensure the area at the bottom of the filter has as much air space as possible for the solids to accumulate. You can do this with nursery seedling trays propped up on rocks.

Which rocks are best for beneficial bacteria?

Any type of rock is fine, but scoria or lava rocks are good options as they are lightweight and have pockets for beneficial bacteria. Bacteria is highly adaptable and will colonise and thrive on most rocks.

The removal of the solids with even water flow over the beneficial bacteria will purify the water. From here, we need to remove the build-up of nitrate, which can be done by using plants!

You need to keep your nitrate level under 40ppm, and plants love nitrate, which is essentially plant food.

What type of plants?

Marginal pond plants like Louisiana iris, canna lilies and rushes are best. Just try to avoid plants with roots that penetrate the area where solids accumulate.

A small pond filter system can be a great spot for lettuce, mint and strawberries or anything else where roots can be constantly wet.

Final step: Disguising the filter

Depending on whether your filter is above ground or below, you’ll have a couple of different options to disguise it. A below-ground filter is easy because none of it will be visible apart from rocks, gravel and plants. This is the easy way to ensure both a natural-looking pond and filter system.

It is also important to keep in mind that the filter doesn’t need to be right next to the pond. It is easier but not necessary as long as water can be pumped from the pond into the filter with gravity returning the filtered water into the pond.

If you have an above ground bog filter, clad it in wood. You can even make a more formal design with straight lines and edges. Alternatively, you could use bricks, bamboo screens or rocks. Work with anything that fits the design and aesthetic of your pond and the surrounding area, and you are good to go!

DIY pond filter box
Source: Jeremy C via Unsplash

Frequently asked questions

How do you maintain a pond without a filter?

If you want to avoid having a pond filter, there are some things you can do to keep your pond and fish healthy:

  1. Never overstock the pond
  2. Avoid overfeeding the fish
  3. Regularly change the water
  4. Use a UV steriliser
  5. Use Nano Media Reactors
  6. Use pond dye
  7. Make sure you stock the pond with the right plants

What can I use for DIY pond filter media?

There are many different materials that can be used as pond filter media in mechanical filtration. The most common are sponges or specified filter foams. You can also use floss, wadding or filter wool.

Does a small pond need a filter?

While there are some things you can do to keep a small pond clean without a filter, nothing is as effective as using a filtration system. You should choose the filter for your pond based on the size of your pond, as well as the number of fish you have. If you want to avoid a filter, you’ll need to change the water every now and then or use pond dye to keep your fish healthy.

Call in the professionals to bring your koi pond filter back to life

As you can see, building your own DIY pond filter can be quite cheap and easy. However, you may have a pond that has been left to its own devices for too long and needs a little revival before you can even think about a new filtration system.

If that is the case, you are in luck. You can find a range of Taskers who are pros when it comes to garden pond maintenance. They can help ensure your pond is clean and healthy, full of happy little fish. You can also engage some cleaning experts and Taskers who can help with garden planting to make sure your outdoor areas look their best.

If you are new to the world of ponds and still in the research phase, read our blog all about backyard ponds. You’ll find a range of helpful information there. Ponds are wonderful additions to any garden area. They let you bring the calming feel of water into your space, provide a home for some pet fish (like the stunning Koi) and create a new dynamic in the nature-filled areas of your home. We’re pond fans and strongly recommend you bring one into your space today!


Luke Carlino

Luke Carlino is a freelance copywriter, social media manager and content creator based in Melbourne. He works for a range of clients across a plethora of industries and is also an accomplished musician with solo and band works online.

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