Transforming a chlorinated pool into a natural pool: steps, costs, and pitfalls

Tired of the smell of chlorine, the containers of chemicals and stinging eyes, you look at your pool and wonder whether it could become a natural swimming pool, with living water and plants. The good news is that most in-ground pools are suitable for this, and a good part of the existing installation can be reused. The conversion always begins with an honest assessment of the structure, lining and plant room, followed by a choice between two main options: adding a planted regeneration zone, or replacing chemical treatment with compact biological filtration. Here is how to decide, what it costs in broad terms, and the pitfalls to avoid.

Can the existing pool be converted

Before dreaming of water lilies, you need to assess the situation. Four points determine feasibility and cost.

A lightweight above-ground kit pool or a self-supporting pool is not really suitable for conversion: its structure is not designed to remain full all year with plants and a bottom that will accumulate some sediment. By contrast, masonry pools, in-ground panel pools, or polyester shell pools are good candidates.

The final, often overlooked point of the assessment is the chemical history. Copper-based algaecides, copper-silver ionizers, and certain copper heat exchangers leave metals that are toxic to plants, zooplankton, and amphibians. If the water contains them, it must be completely drained and rinsed before any biological system is started.

Scenario 1: adjoining regeneration zone

This is the approach closest to natural swimming. The pool is kept as the swimming area, and a shallow planted area is created beside it or as an extension, using an EPDM liner. Water from the pool is pumped into this area, flows slowly through it, then returns via an overflow or a sheet of water.

1. Size the area: approximately the same surface area as the pool for a very natural approach, or about one third to one half with supplementary filtration.
2. Excavate and waterproof the area in tiers from 0 to 60 cm, over geotextile felt and an EPDM liner.
3. Install the circulation system: draw water through the existing skimmers, use an energy-efficient pump, distribute it through the planted area (flow-through planted filtration), and return it by gravity.
4. Plant the tiers: marginal plants, floating-leaved plants, and submerged oxygenating plants.

Advantages: very natural appearance, biodiversity, minimal technical equipment. Disadvantages: it requires space, excavation and waterproofing account for most of the budget, and the result takes one to two seasons to stabilize. To choose the flow direction, see downflow or upflow planted filtration.

Scenario 2: compact biological filter

When the garden does not allow for adding a planted area, the pool can be left as it is and the technical room transformed. The sand filter and chlorine are replaced by biological filtration, often supplemented with a medium that binds phosphates. This is then more accurately called a biological pool: the water is alive and chlorine-free, but the pool retains the appearance of a swimming pool.

• Replace the sand filter with a biological filter sized for the volume, or with a small planted filtration bed in a nearby container.
• Replace the original pump, which is often powerful and energy-hungry, with an energy-efficient pump capable of running 24 hours a day.
• Remove the electrolyzer, chlorine and pH dosers, and ionizer.
• Add if needed a few submerged planters along a wall, or a small planted area in a corner behind a low wall, to introduce life.

Advantages: no excavation, quick construction, preserved swimming area. Disadvantages: more technical equipment and filter maintenance, and greater dependence on phosphorus control.

Lining: keep or replace the liner

The light-blue liner of conventional pools is not a technical obstacle, but it has two drawbacks in a natural pool: it gives the water a somewhat artificial “pool-like” appearance, and it makes the slightest biofilm visible, that fine brown-green deposit that is normal in living water. Many owners therefore take advantage of the conversion to switch to a darker color: anthracite gray, green, or sand.

A dark color warms the water slightly more in the sun: this is pleasant in spring, but needs monitoring during heat waves. A natural pool does not like water remaining above approximately 25 °C for long periods.

Reuse the technical equipment room

This is often where the savings lie. Most of a pool's hydraulic components can be reused:

• Skimmers and main drain: keep them; they ensure water circulation. An additional floating skimmer is useful if the wall skimmers are poorly positioned in relation to the wind.
• Return inlets: keep them, or redirect them toward the regeneration zone.
• Pipes and valves: reusable, provided they are rinsed to remove product residues.
• Pump: often needs replacing. A conventional pool pump consumes a lot and should not run continuously; a natural pool needs permanent circulation at a lower flow rate.
• Heating: a heat pump can remain, but with moderate use.

Our article which pump for a natural pool helps you choose the right model.

Plants and wildlife

In scenario 1, the regeneration zone accommodates most of the plants. In scenario 2, a few submerged planters or a small planted area in a corner add life and provide some competition for algae. Favor compact native species: yellow flag iris, sedges, water mint, marsh marigold, small water lilies, coontail, and spiked water milfoil for submerged planting. Our guide which aquatic plants to choose for your natural swimming pool provides details on suitable combinations.

Wildlife arrives on its own, often as early as the first summer: dragonflies, backswimmers, water striders, and sometimes newts or frogs. They are good indicators of healthy water. However, do not introduce fish: they disturb the bottom, consume the zooplankton that grazes on algae, and enrich the water. Also consider a small ramp or gently sloping bank so animals that fall into the water can get out.

The most common pitfalls

Conversions that disappoint have almost always run into one of these issues. Anticipating them costs little; correcting them afterward is expensive.

• Start without draining, while the water still contains chlorine, stabilizer, or copper. The plants turn yellow, the bacteria fail to establish, and people wrongly conclude that “it doesn’t work.”
• Keep the old pump and run it for a few hours a day, as with a chlorinated pool. A biological swimming pond needs continuous circulation.
• Fill it with phosphate-rich water without testing it first. The initial nutrient load feeds algae throughout the first season.
• Make the planted area too small due to limited space, without compensating with biological filtration. The pool then remains halfway between a swimming pool and a natural swimming pond.
• Return to chemicals at the first sign of cloudy water. Shock treatment destroys the developing biological balance and resets the process.
• Overlooking the environment: a hedge that sheds its leaves into the water or a fertilized lawn whose runoff flows into the pool can undermine even the best systems.

Comparative budget

Every project is different, and prices vary widely depending on access, region, and the proportion of self-construction. The estimates below are intended to compare the two scenarios with each other, not to prepare a quote.

In practice, scenario 2 often costs a few thousand euros in equipment, while scenario 1 can cost several times that amount once excavation and liner area increase. In both cases, stopping chemical products and replacing an energy-hungry pump permanently reduces annual costs.

First summer: what changes

This is when many owners start to have doubts. Natural water does not look like chlorinated water, especially during the first season.

• A biofilm appears on the walls and bottom: this is normal; brush and vacuum it. It is part of the ecosystem.
• Filamentous algae may develop in spring, before the plants are well established. Remove them by hand or with a net on a regular basis.
• Transparency varies: after a hot spell or a period of heavy swimming, the water may become slightly cloudy for a few days.
• Temperature matters: during a heatwave, increase circulation and avoid heating.
• Habits change: shower before swimming, use sunscreen in moderation, and do not let leaves accumulate.

A bacterial starter culture helps quickly establish beneficial flora; our article bacteria in natural swimming pools and constructed wetlands explains which ones to choose. Also have the water tested regularly, especially for phosphates and nitrates: these are what signal algae problems before they become visible.

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Frequently asked questions

— Can any swimming pool be converted into a natural swimming pool?

Most sound in-ground pools are suitable: masonry, panel, or fiberglass pools. Lightweight above-ground pools and structurally damaged pools are poor candidates.

— Does the pool need to be emptied for conversion?

This is strongly recommended to remove residual chlorine and any metals and to inspect the structure. The pool is then refilled with low-phosphorus water.

— Can the original blue liner be kept?

Yes, if it is in good condition. A darker color gives it a more natural appearance and better conceals the biofilm, but the change can wait until the liner reaches the end of its service life.

— How much does it cost to convert a swimming pool into a natural swimming pool?

It mainly depends on the scenario. Replacing the treatment system with a biological filter generally costs much less than creating an adjacent regeneration zone, which involves excavation, a liner, and plants.

In summary

• Diagnose first: structure, lining, depth, traces of copper.
• Scenario 1: adjacent regeneration zone, very natural but space-intensive.
• Scenario 2: compact, fast biological filter with lower investment cost.
• Reuse skimmers and pipes; replace the pump with an energy-efficient model whenever possible.
• First summer: biofilm and filamentous algae are normal; balance is established over time.

Last updated: October 3, 2026.