Why install a bottom drain for your pool?
In a natural swimming pool, the skimmer collects floating debris, but it cannot see what sinks. The main drain draws water from the lowest point and carries light sediment to the filtration system before it can accumulate. The two complement each other. The main drain is not mandatory, but it is installed during construction—the ideal time to consider it.
Today, I would like to explain why we recommend main drains in addition to skimmers in natural swimming pools.
Two different roles
The skimmer works at the water line. Biotop, a natural pool manufacturer, simply summarizes its role: to remove floating particles from the surface. Anything that slips past it eventually sinks: pollen weighed down by water, leaf fragments, dust. Once at the bottom, it is no longer the skimmer's concern.
The main drain takes over. Placed at the lowest point, it draws water where particles settle. Our article on the number of main drains in a pool reminds us that it mostly complements the skimmer. It also helps circulate the bottom, which the skimmer does not do.
What the main drain changes in a natural swimming pool
A chlorine pool oxidizes some of the organic matter. A natural swimming pool relies on something else: the planted zone, microorganisms, and water circulation. Anything that remains at the bottom slowly decomposes there, releasing nutrients into the swimming water.
This is where the main drain makes sense. By drawing this sediment into the system, it carries it to the filtration and purification zone, where it is captured and treated. The plants and bacteria then receive the organic matter in the right place, rather than letting it settle on the floor of the swimming area. The resulting reduction in algae growth is due to this: less available organic matter means greater biological balance.
Circulation is equally important. According to a professional pool publication, it supports biological processes and distributes nutrients and oxygen throughout the pool. Biotop, for its part, separates two systems: one cleans the surface, while the other removes organic compounds. This is the same logic as the skimmer-and-main-drain duo, even though each manufacturer has its own setup.
To support this process, Aquipond BactoSwim provides bacteria that promote the decomposition of organic matter in the pool and its lagoon zones. First, check your KH: if it is below 7, mineralize before treating, for example with Aquipond KH+.
How it fits into the system
The principle described by Gardenvisit is simple: a pump draws water from the base of the swimming area through grates on a hard bottom or porous pipes under gravel, then returns it to the edge of the regeneration zone. The skimmer is added to this circuit to keep the surface clean.
- Think about it before you dig. The main drain is installed before the concrete or before the liner it will be clamped to.
- Choose a bottom that slopes toward it. A slight slope guides sediment. On a flat bottom, it remains where it falls.
- Plan the pipe routing. It must connect to the pump or filter without high points or unnecessary bends.
- Add a valve. It allows you to adjust the distribution between the main drain and skimmer and isolate the line for maintenance.
- Test it with water. Check the seal of the flange before closing or planting it.
For detailed installation, our guide on installing a main drain describes the connections, the liner flange, and common mistakes. The pump is then chosen based on the volume: see which pump for a natural swimming pool.
Main drain for swimming 50/63 mmUV-treated ABS, polymer compatible with natural swimming pools, stainless steel hardware. Seven colors.View productSwimSkim 50 - OaseFloating skimmer for ponds up to 50 m², from 40 cm deep.View productAquipond BactoSwim 5 kgBacteria for natural swimming pools from 25 to 50 m², to be integrated into seasonal maintenance.View product
Swimmer safety
A water intake at the bottom of a pool is not a minor detail. U.S. authorities, who have studied the issue extensively, speak of the risk of entrapment by suction: the Michigan Department of Environmental Quality reminds us that a body can be held by the suction force of a pump. These regulations primarily target public pools, but the physics remains the same at home.
Caution. Michigan recommends more than one intake so that a second remains free if one is obstructed. The American guide Pool Safely distinguishes multiple intakes spaced at least 3 feet apart, or about 91 cm. A broken, poorly secured, or missing grate should be avoided. Have any setup inspected by a professional, especially if the pump draws directly from the main drain, and turn off the pump if in doubt.
We also recommend an anti-vortex device and a sturdy grate that can support the weight of swimmers. Children should not play near the main drain while the filtration system is running.
What the main drain does not replace
A main drain does not clean the entire bottom. Areas without current, corners, and steps retain their sediment. The Pondovac 5, whose product sheet lists natural and biological pools, remains useful for these areas, and our guide on pool sludge vacuums explains how to vacuum properly.
Leaves remain the primary concern in autumn. Guide-Piscine reminds us that leaves and hair do not disappear in the purification zone and must be removed. Empty the skimmer basket, use the skimmer net, and check that the main drain grate remains clear. For the cold season, our article on winterizing a natural swimming pool lists mistakes to avoid.
Finally, monitor your water rather than the calendar. An analysis helps identify an imbalance before it becomes visible, and our natural swimming pool guide places the main drain within the overall project.
Frequently asked questions
Is a main drain mandatory in a natural swimming pool?
No. Some natural swimming pools do without one, and Gardenvisit also presents systems using porous pipes under gravel. We recommend it because it draws water from the lowest point, where sediment accumulates.
Can a main drain be added to an existing pool?
It is difficult, as the main drain is installed during construction, before the concrete or liner. On a pool already filled with water, reinforce the skimmer and bottom suction instead, then ask us for advice based on your structure.
Should you install a main drain and a skimmer, or just one of the two?
The two complement each other: the skimmer treats the surface, while the main drain treats the bottom. If only one is to be installed during construction, consider that the skimmer can be added more easily later than the main drain.
How many main drains should you plan?
It depends on the surface area and the pump. Our article on the number of main drains in a pool provides our guidelines. For safety, two intakes spaced apart are better than a single intake on a direct-suction pump.
Is a main drain sufficient to limit algae?
No. It helps remove organic matter from the bottom. Less sediment means fewer available nutrients, and it is this reduction that helps maintain the pool's balance, along with the planted zone and regular circulation.
Are you planning a natural swimming pool or unsure about the system? Send us your dimensions.
Contact an advisorWater analysis
Read also
For the complete project: Natural swimming pool: the guide to designing and maintaining it
READ THE ARTICLESources and references
- Biotop: Planning a natural pool
- Gardenvisit: Swimming pond construction
- Pool & Spa Marketing: A review of five natural swimming pool designs
- Guide-Piscine: The purification or filtration zone of your natural swimming pool—how it works
- Michigan EGLE: Main drain, frequently asked questions
- Pool Safely: Virginia Graeme Baker Act, frequently asked questions
- Play It Koi: Bottom drain vs skimmer
- Product sheet: Main drain for swimming 50/63 mm
- Product sheet: Aquipond BactoSwim
Last updated: November 4, 2026.
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