Natural swimming pool: the guide to designing and maintaining it
A natural swimming pool, or biological swimming area, is chlorine-free: the water is continuously purified by a planted lava rock filtration zone, where bacteria and roots live and consume nutrients. Success depends on a few key sizing guidelines, careful start-up, and regular maintenance, far more than on any product. This guide brings together the guidelines our experts have used since 1998, in the order you will need them.
≈ 30% of the water volume in the filtration zone
4 to 5 plants per m² of filtration zone
60 cm of lava rock on a grating
1 h for all the water to pass through the filter (2 h maximum)
How does a natural swimming pool work?
The principle is based on a single loop. A pump draws water from the swimming area, passes it through the filtration zone, and then returns the purified water to the swimming area. In the filtration zone, the lava rock provides a large surface area for bacteria, which break down organic matter. The plants, in turn, absorb the nutrients that algae need to grow.
We distinguish between a natural swimming pool, with a visible planted zone, and a biological swimming area with a discreet biofilter, which looks like a conventional pool. The principle remains the same in both cases: a purification volume, sufficient flow rate, and a balance between nutrients and bacteria. If you are starting with an existing pool, our article on converting a pool into a garden pond details the possible options.
A natural swimming pool is not "treated"; it is balanced. Every design choice (filtration zone volume, flow rate, plants) matters more than any product added later.
Sizing the swimming area, filtration zone, and flow rate
Here are the guidelines validated by our experts for a biological swimming area project:
- Filtration zone: approximately 30% of the total water volume.
- Plants: 4 to 5 plants per m² of filtration zone.
- Filter: lava rock 60 cm high, placed on a grating.
- Flow rate: all the water must pass through the filter in 1 hour, 2 hours maximum.
- Drainage: 1 Drain Connector per 30 m² filtration zone, in the 60 cm or 100 cm version, with downflow.
In practical terms, a 20 m² filtration zone accommodates 80 to 100 plants. A 60 m² filtration zone requires two Drain Connectors. For a pond with a total of 100 m³ of water, aim for approximately 30 m³ of filtration zone. If you do not yet know your volume, our article on calculating the volume of a round pool gives you the method.
The flow rate is simply calculated: for 60 m³ of water, the pump should circulate 60 m³ per hour at best, 30 m³ per hour at minimum. Always read the flow rate on the pump curve at the actual installation height, including pipe losses. Our selection of pumps for natural swimming pools is designed for these uses.
The natural swimming pool calculator in our calculator hub performs these calculations based on your dimensions: you get numerical guidelines and the right equipment, without a spreadsheet.
Warning: a filtration zone that is too small or a flow rate that is too low will result in more maintenance. It is better to correct it on the plans than to reopen the construction site.
Building the lava rock filter, step by step
The lava rock filter is the heart of the system. Here is the assembly order we recommend:
- Prepare the waterproof base of the filtration zone, separate from the swimming area.
- Install the drains and the Drain Connector under the future filter: one Drain Connector per 30 m² of filtration zone, in the 60 cm (the most common, under 60 cm of lava rock) or 100 cm version.
- Install the grating, which supports the rock and allows water to circulate freely underneath.
- Build the lava rock in three layers, 60 cm high: large rocks at the bottom on the grating, then 1/3 of 16-32 mm, then 1/3 of 8-16 mm.
- Plant 4 to 5 plants per m² in the thin top layer.
- Connect the pump and check that all the water passes through the filtration zone in 1 hour, 2 hours maximum.
Here is what we recommend for this step, and the solution we use in our ponds: complete downflow drainage and high-quality lava rock that is already largely cleaned. For a large volume, big bags are more practical than bags.
Pondtechnics Drain Connector, 60 or 100 cmComplete set for downflow drainage under the filtration zone: drain connector, 50 m pipe, sleeves, and cover. Plan for 1 per 30 m² of filtration zone.From €659.00 · See product →
16-32 mm pozzolanic lava rockAvailable in a 20 kg bag, a small 0.33 m³ big bag, or a large 1 m³ big bag. The 8-16 mm and large rocks complete the three layers.20 kg bag: €24.95 · See product →
For the other two layers, find the 8-16 mm lava rock and the 5-8 cm lava rock for the large rocks at the bottom. Other drainage and filtration accessories are gathered in the filtration zone solutions collection.
Do not mix the particle sizes: each layer has its role in distributing the water throughout the height of the filter. Build them in order, from largest to finest.
Choosing and planting the plants in the filtration zone
Plants are not just decoration: they absorb nutrients and provide oxygen to the substrate. Allow 4 to 5 plants per m² of filtration zone, mixing the depths.
| Zone | Water depth | Examples |
|---|---|---|
| Bank and wetland zone | 0 to 10 cm | Water mint, yellow flag iris, bulrush |
| Shallow water | 10 to 30 cm | Water lilies, bulrush, rushes, flowering rush |
| Submerged plants | Beyond 50 cm | Lesser water-parsnip, water milfoil, water crowfoot, waterweed |
Submerged plants absorb nutrients through their thin leaves and compete with filamentous algae. Their rapid growth requires regular trimming, which exports these nutrients from the system. Browse the entire collection of plants for filtration zones and phyto-purification and read our article on choosing aquatic plants.
Plant from May to September: water that is too cold slows down recovery, and May-June remains the best period. Use a planting basket and aquatic potting soil, remove the plant from its pot, loosen the roots, center it in the basket, and tamp down.
Prune as winter approaches, as soon as the leaf turns yellow, and remove green waste from the pond. Left in the water, they would release the nutrients your plants have just captured.
Starting the pool: bacteria and minerals
A new filtration zone, or one restarted in spring, benefits from a biological boost. As soon as the water reaches 15 °C, add bacteria specific to biological swimming areas. With BactoPool (Aquatic Science), the starting dosage is 1 kg per 10 m³ of water, then a half dose every 2 to 3 months, or 2 to 3 times a year. BactoSwim is dosed according to the surface area of the filtration zone: 25 g per m² of filtration zone per application, 2 to 3 applications per season as soon as the water reaches 15 °C. Follow the sheet for the chosen format.
A few steps improve the result. Spread the powder on the surface of the planted substrate, where the water circulates well, and avoid areas of strong current or near the swimming area: this insoluble powder can slightly whiten the water for a few days. Also avoid vacuuming the treated areas for several weeks. Before starting, remove as many algae as possible by hand. You will find the entire range in the bacteria for natural swimming pools collection and explanations in our article on filtration zone bacteria.
Minerals are just as important as bacteria. Water that is too soft, for example supplied with rainwater, becomes unstable and its pH varies more. Measure the GH, KH, and pH. If they are too low, MinerAll In One provides the minerals: 2.5 kg per 30 m³, with 2 applications per month at start-up or after an imbalance, then 1 per month for maintenance. To adjust KH alone, consult the collection increasing KH in natural swimming areas.
Maintaining the natural swimming pool all year round
The maintenance of a biological swimming area follows the seasons. This calendar summarizes the main steps.
In summer, warm water holds less oxygen. Circulating or aerating the filtration zone and deep areas supports the bacteria: see our selection of pond aerators. When the walls become slippery or the bottom silts up, Boost & Clean helps loosen deposits: the dosage is 25 g per m³ as soon as the water exceeds 15 °C, with at least 15 days between two applications. The waste that rises is then removed with a net.
A light biofilm on the walls is part of a living pond: before cleaning everything, read our article on slippery walls. For the sludge at the bottom, a mud vacuum is the right tool.
Your seasonal checklist:
- Regularly remove leaves and debris with the skimmer and net.
- Measure the pH, GH, and KH at start-up, then after heavy rains or a major water top-up.
- Add a half dose of bacteria every 2 to 3 months.
- Vacuum the deposits from the bottom of the swimming area.
- Prune the plants as winter approaches and remove green waste.
- Check that all the water still passes through the filter in 1 to 2 hours.
- Clean the pump and its pre-filter with OASE PumpClean, never with bleach.
Warning: no chlorine, bromine, copper, or improvised treatment in a biological swimming area: a chlorine shock can destroy useful bacteria. If you suspect cyanobacteria (blue-green film, foam, strong odor), suspend swimming and have the phenomenon identified.
Cloudy water, algae: reading the signs without breaking everything
We never try to make algae disappear all at once: we limit them by rebalancing nutrients, bacteria, oxygen, shade, and plants. Filamentous algae appear mainly in spring and autumn, when light levels drop and the nitrate level is high.
| What you see | Probable cause | First step |
|---|---|---|
| Green filaments on the walls | Excess nutrients, decreasing light | Remove by hand, prune plants, restart bacteria |
| Green or cloudy water | Broken balance or insufficient flow rate | Check the flow rate, then GH, KH, and pH |
| Slippery walls, silty bottom | Accumulated organic matter | Vacuum, remove waste, Boost & Clean if needed |
| Unpleasant odor | Oxygen-poor areas | Aerate, remove debris |
Our guide to algae by pool type specifies what to avoid in a biological swimming area. Doubt about the cause? Our free water analysis by our experts allows you to make a diagnosis before adding anything.
Frequently asked questions
What filtration zone size to plan for a natural swimming pool?
Our experts' guideline: the filtration zone represents approximately 30% of the total water volume. For a pond with a total of 100 m³, this gives a filtration zone of approximately 30 m³. The natural swimming pool calculator refines this guideline based on your dimensions.
How many plants to put in the filtration zone?
Allow 4 to 5 plants per m² of filtration zone. For 20 m², this represents 80 to 100 plants, to be distributed between bank plants, shallow water plants, and submerged plants.
What height of lava rock and how many Drain Connectors?
The lava rock filter is built 60 cm high, on a grating, in three layers: large rocks at the bottom, then one third of 16-32 mm, then one third of 8-16 mm. Plan for one Drain Connector per 30 m² filtration zone, in the 60 cm (the most common) or 100 cm version, with downflow.
How fast should the water pass through the filtration zone?
All the water must pass through the filter in 1 hour, 2 hours maximum. For 60 m³ of water, this requires at least 30 m³ per hour, to be read on the pump curve at the actual installation height.
When and how to add bacteria?
As soon as the water reaches 15 °C, add 1 kg of bacteria per 10 m³ of water (BactoPool, Aquatic Science), spread on the surface of the planted substrate, where the water circulates well. Then, a half dose every 2 to 3 months, or 2 to 3 times a year.
My water is cloudy or algae appear: what to do?
Remove the algae by hand, prune the plants, check the flow rate then the GH, the KH, and the pH, and restart the balance with bacteria. We limit algae by rebalancing nutrients, bacteria, oxygen, shade, and plants. Never add chlorine or improvised treatment in a biological swimming area.
For more information
- How to convert a pool into a garden pond?
- Bacteria in natural swimming pools and lagoon systems: which ones to use?
- Which aquatic plants to choose for your natural swimming pool?
- How to build your own natural or ecological swimming pool
- Algae in wooden, salt, hydrolysis, or biological swimming pools
- How to remove sludge from a swimming pool?
- Sludge vacuum for swimming pools: which model to choose
- Plant-filtered pond: designing a planted zone that works
Unsure about the sizing of your lagoon system?
Our experts, in business since 1998, analyze your water free of charge and provide advice.
Free delivery on orders over €149 (excluding bulky items).
Are you keeping a chlorine swimming pool? Our comprehensive guide will walk you through it step by step: Chlorine swimming pool maintenance: the complete guide.
Les produits conseillés par nos experts
Voir toute la sélection
Aquipond BactoClay - Montmorillonite clay 2 kg for koi ponds
€41,95
Voir le produit
Minerall in One Aquipond - Increases GH & KH simultaneously - 2.5KG to treat 30M³
€51,45
Voir le produit
Sera Aqua-Test Set - The essential analysis kit for ponds and aquariums
€36,34
Aquipond KH+ 1kg - Increases Carbonate Hardness (KH)
€20,95
À lire aussi
Tous les conseils
Maintaining your pool with chlorine or salt hassle-free: the complete guide
12 min de lecture
Too much chlorine in the pool: 6 ways to lower the level, from the gentlest to the fastest
12 min de lecture
Removing pool flocculant: vacuum up the sediment, rinse the filter, and correct an overdose
11 min de lecture
Leave a comment