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11 Reasons to Use a Bottom Drain in a Koi Pond

by Chris Thomas January 08, 2024 5 min read

11 Reasons to Use a Bottom Drain in a Koi Pond - Kitsu Koi

When building a koi pond, hobbyists must choose whether to use gravity fed system with a bottom drain, or a pump fed system. The cheapest way to build a pond is to build it right the first time, so making the right decisions at the planning stage can save you money. By choosing the right system, and matching your pond equipment to compatible components, you can improve your enjoyment of the hobby.

In this post we will explain 11 examples of why we think you should use a bottom drain in your pond build.

What is a Bottom Drain?

A bottom drain is a sump that is fitted at the lowest part of the pond. Generally two types of drain are available

  • The standard bottom drain, made from injection moulded black plastic, and featuring a 110mm socket
  • The aerated bottom drain, fabricated from PVC, with a 4” PVC Pressure socket, and a 45 degree 1” fitting for an airline. Inside the drain is a stem that carries air to the drain lid which has a perforated rubber diffuser. The exact arrangement varies by manufacturer.
 A Standard bottom drain for a koi pond An aerated bottom drain for a koi pond


On the left, the standard bottom drain. Black in colour, they are injection moulded and usually have a 110mm socket. On the right, the aerated bottom drain. Fabricated from PVC pipe, PVC sheet, and PVC fittings, they feature an air diffuse for a dome. 

The working principles of both drains are essentially the same. Located at the bottom of the pond, they feed water directly to the filter via a 100mm/4” pipe. Bottom drains can be used on ponds lined with fibreglass, or pond liner when used with a clamping ring. People often worry about cutting a liner when using a bottom drain, but with the correct sealant and a liner clamp, you shouldn’t have any problems sealing the drain and liner.

 Image showing the layout of a gravity fed filter
Image showing the layout of a pump fed filter


The above schematics show the difference between a Gravity fed (top) and a pump fed (bottom) installation of the Evolution Aqua Nexus 220+/Nexus 320+ filter system. In the Gravity system, a bottom drain is used with 110mm/4" pipe to deliver water to the filter, and then a pump circulates the water back to the pond via a UV. 

For aerated drains, the 1” pipe feeds the air from an air pump to the sump, and via the stem to the diffuser.



Detail shot of the inside of our Aerated Bottom Drain. The air is supplied to the dome via a 1" pipe, that feeds direct in to the stem for the drain lid/diffuser, minimising the risk of entrapment by additional air lines in the sump/drain line. 

At Kitsu Koi, all our ponds are gravity fed with a bottom drain.

Here are our 11 reasons why you should use a bottom drain in your koi pond

  1. Running Costs - moving water to the filter by gravity means the water travels from pond to filter with the most efficiency. A pump fed pond requires the filter to be placed above the pond, which means lifting water above the natural water level. Lifting water comes at a price - it requires more energy and therefore costs more money to achieve the same flow rate. 
  2. Clutter Free Pond - A pump fed pond will always have a pump and hose/pipe inside the pond. This creates an obstruction when catching fish, and also spoils the visual appeal of the pond. Having an aerated drain further reduces clutter as you won’t usually require extra airlines and air stones in the pond. 
  3. Less Pond Maintenance - As the bottom drain is situated at the lowest part of the pond, any negatively buoyant debris will be easily collected by the drain, reducing the need for pond vacuuming. 
  4. Access to Better Filtration - Gravity filters tend to be better filters. That’s to say high quality filters tend to be made for gravity filtration, with 110mm/4” inlets for higher flow rates, better use of filter media’s, and good mechanical filtration for better clarity and water quality.
  5. Water Quality - In a pump fed system, debris has to pass through a pump. While most pumps won’t work quite like a blender, the particles delivered to the filter will be smaller and harder to capture. The bottom drain and large diameter pipe deliver the waste to the filter in larger sizes, which are easier to trap than smaller particles. Removing as much waste as possible results in less pollution from decaying debris, and better water clarity for your viewing pleasure. 
  6. Equipment efficiency - In a gravity fed system, the pumps and UV’s are located after the filter and will only come in to contact with clean water. This prolongs the life of impellers and quartz sleeves, reducing the maintenance of your pond equipment. Having the UV and pump mounted externally also makes access easier when it comes to replacement. 
  7. Flow - In a koi pond, having a good flow can promote better quality water by ensuring there are no dead spots. The bottom drain is taking the water from the bottom of the pond to improve the circulation of water. This effect is enhanced with the use of an aerated drain. If you combine this with mid water returns, you have vertical and horizontal flow of water in the pond. 
  8. Simple Installs - On a pump fed system, you can usually only pump to one filter, and if you need any secondary filtration, you will need a carefully balanced pump, which can be a headache. With a bottom drain and gravity system, you can add to your filtration easily, usually with the need for secondary pumps.
  9. Visual Appeal - Siting the filter at the water height of the pond means you don’t have to have potentially ugly filter boxes sat on the pond wall.
  10. Safety - A common approach with a pump fed system is to use some kind of flexible hose. Some types are prone to splitting, or even coming loose over time. Should your flexi hose fail on a pump fed system, the pump will empty the pond. This is not possible on a gravity fed system. Some will argue that a bottom drain can fail, but if it’s encased in concrete in the pond base, you have bigger issues to care about! 
  11. Winter - A gravity fed pond filter fed from a bottom drain will not overflow if the pond freezes. If a pump fed filter is a non pressurised filter and it freezes, the pump will just overflow the filter box leading to an empty pond. 

Hopefully you are now pro-bottom drain and can see the benefits to your pond, fish, and koi hobby. We have seen all the excuses for people not using a bottom drain for a koi pond, and not one of them can’t be overcome, leading to most if not all of the above benefits. We simply won’t build a pond without one - in our premises or for customers.

If you’re in need of a bottom drain, and all the rest of the components to build a quality koi pond, then start your journey with Kitsu Koi. Below you will find links to our popular bottom drains, pipework, and valves needed to start your build. Then you will need to choose your filter - we recommend a visit if you can to see our filters in action, or call for more information.

Aerated Bottom Drain Grey PVC Pressure Pipe for pond building PVC PRESSURE 90 DEGREE PLAIN ELBOW/BEND Ever-Weld P-194 Solvent Cement for jointing of PVC Pressure Pipe and Fittings
 Aerated Bottom Drain Grey PVC Pressure Pipe PVC Pressure 90 Degree Elbow Ever-Weld p-194 Solvent Cement

Chris Thomas
Chris Thomas

Chris has been keeping koi for over 25 years, and dealing in koi for 15 years. Travelling to Japan to select new stocks is the favourite aspect of the business, closely followed by being on the tools pond building.

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