Biofilm and slippery walls in a natural pool: do you really need to clean everything?

Do you run your hand along the wall of your natural pool and find it slightly slippery? Does a thin greenish or brownish layer appear on the bottom despite perfectly clear water?

This is not necessarily a sign of a dirty pool.

In a natural pool or swimming pond, submerged surfaces are quickly colonized by a multitude of microorganisms. They gradually form what is known as a biofilm, or more broadly a periphyton when considering all the organisms living on submerged surfaces.

The instinctive reaction would be to try to remove this layer completely.

However, in a natural pool, the right approach is different:

the goal is not to sterilize the walls, but to prevent biofilm and organic matter from accumulating excessively.

That is the difference between cleaning a traditional pool and maintaining a living aquatic ecosystem.

What is biofilm in a natural pool?

A submerged wall is never truly inert.

After a few days or weeks, bacteria, microalgae, microscopic organic matter, and other organisms naturally appear and gradually colonize the surfaces.

This community forms a thin biological layer.

It can be found on:

the walls of the swimming area, the bottom, the stones, the gravel in the regeneration zone, decorative elements, and practically all constantly submerged surfaces.

In the biological filtration zone, this phenomenon is particularly interesting: the microorganisms living on the media contribute to the pool’s biological processes.

A natural pool completely free of biofilm would therefore be rather… unnatural.

Why do the walls sometimes become very slippery?

However, it is important to distinguish normal biofilm from excessive buildup.

When the walls become very slippery, a substantial layer appears on the bottom, or the robot fills up extremely quickly, this often indicates that a large amount of organic matter is circulating or accumulating in the pool.

Leaves, pollen, dust, plant residue, microalgae, and other particles eventually settle.

Over time, this material feeds and thickens the biofilm.

Temperature also plays an important role. As the water warms up, biological activity increases sharply.

That is why a natural pool can look perfectly clean in spring, only for its walls to become much more slippery a few weeks later.

Should the biofilm therefore be cleaned?

Yes… but intelligently.

It is generally neither necessary nor desirable to seek a completely sterile surface.

In the swimming area, however, it is entirely reasonable to want to keep a bottom and walls that feel pleasant to the touch.

The principle is therefore to mechanically control excess biofilm while preserving the pond's overall biological function.

And above all, to actually remove organic matter from the system.

Loosening dirt only to let it settle again a few meters away does not accomplish much.

This is where combining mechanical cleaning with biological management becomes particularly interesting.

Our 3-step method for a cleaner natural pool

At Foudebassin.com, we prefer to think in terms of three complementary actions:

1. Loosen significant accumulations.

When walls, stones, or certain areas of the bottom are heavily laden with organic matter, Aquipond Boost & Clean can be used as a one-off treatment.

Boost & Clean is suitable for natural pools and helps loosen organic deposits and excess biofilm present on surfaces.

The recommended dosage is 30 g per m³ of water, with water above 15 °C.

Filtration and circulation must remain effective during treatment.

Some of the material may then come loose or rise to the surface: this is precisely when it needs to be removed.

→ Discover Aquipond Boost & Clean

2. Physically remove what has been loosened

This is probably the most important step.

Organic matter removed from the pond can no longer decompose at the bottom.

For a large natural pool, regularly cleaning several dozen square meters manually quickly becomes burdensome, however.

A robot specifically designed for swimming ponds can then make a huge difference.

The AquaForte BioPool Robot Plus is specifically developed for natural pools and biopools.

Its specialized brushes work in particular on stubborn dirt and biofilm, while its suction collects particles present on the bottom and accessible surfaces.

The goal is therefore not simply to make the bottom look nicer.

The robot makes it possible to regularly remove organic matter from the ecosystem.

And the less of this material remains in the water, the less it can contribute to the formation of new deposits.

→ Discover the AquaForte BioPool Robot Plus

3. Then support the biological balance

Once most of the sediment has been removed, the work does not stop there.

A natural pool functions through its biological activity.

That is precisely the role of Aquipond BactoSwim, our blend of natural bacteria specifically intended for natural pools and swimming ponds.

BactoSwim supports the biological breakdown of residual organic matter and supports the biological functioning of the pool and lagoon areas.

It therefore does not replace the robot.

And the robot does not replace biological activity.

The two approaches complement each other:

the robot removes what can be removed physically; the bacteria support the biological work on what remains in the ecosystem.

→ Discover Aquipond BactoSwim

The trap: wanting a natural pool to be as sterile as a chlorinated pool

This is probably the most common mistake when discovering natural swimming.

A natural pool is not a traditional pool from which chlorine has simply been removed.

It is an ecosystem.

A slight biological film on some surfaces is therefore entirely consistent with how it works.

The problem tends to begin when the buildup becomes so extensive that:

• the walls become unpleasantly slippery;

• a significant amount of sediment continuously appears on the bottom;

• the lagoon area becomes heavily laden with organic matter;

• leaves and plant residues decompose directly in the water;

• the robot or filtration system constantly collect large quantities of waste.

In this case, it is better to address the overall organic load rather than simply try to hide the problem.

Why a conventional robot is not always ideal

Not all natural pools have the same lining as a traditional pool.

EPDM, natural stone, slate, steps, submerged ledges, and unusual geometries are common.

The biofilm can also make surfaces more slippery than a traditional liner.

That is why we favor robots genuinely designed for swimming ponds and natural pools.

The BioPool Robot Plus notably features specific brushes designed for the surfaces and dirt found in this type of pool.

Before purchasing, however, remember to check the geometry of your pool: steps, submerged ledges, differences in depth, or planted areas can affect the behavior of any robot.

Our team can advise you based on a few photos of the pool.

How often should you run the robot in a natural pool?

There is no universal frequency.

A pool surrounded by trees and vegetation receives much more organic matter than a pool located in a very open environment.

Frequency also depends on the water temperature, usage, surface area, lagooning, and the amount of pollen or leaves.

Our advice is simple:

do not wait until several weeks of deposits are visible before taking action.

Regular passes generally require less effort than a major cleaning when the walls and bottom are already heavily colonized.

What about lagooning?

Warning: the philosophy is not exactly the same.

In the swimming area, the aim is generally to have a pleasant, visually clean surface.

In the regeneration or filtration area, surfaces colonized by microorganisms directly contribute to biological functioning.

There is therefore no question of systematically scrubbing every stone or piece of gravel.

The main aim is to prevent excessive accumulation of leaves, dead vegetation, and organic matter that could gradually clog the area.

A natural pool does not mean no maintenance. It means thoughtful biological maintenance.

Boost & Clean + robot + BactoSwim: why this combination is worthwhile

These three solutions do not perform the same task.

Boost & Clean is used occasionally when the surfaces are heavily soiled and helps loosen deposits.

The AquaForte BioPool robot mechanically removes some of the matter present on the bottom and walls.

BactoSwim then supports the biological activity of the natural pool and the breakdown of residual organic matter.

This approach seems much more consistent to us than looking for a miracle product capable of doing everything.

In a natural pool, the best results are generally achieved by combining:

prevention + mechanical cleaning + waste removal + biological activity.

Our Foudebassin.com advice

If your natural pool simply has a thin biological film on its walls, but the water looks good and the pool is functioning perfectly, there is not necessarily a problem.

On the other hand, if the surfaces become extremely slippery, deposits build up quickly, or you spend a lot of time manually brushing your pool, it may be worth reviewing your maintenance method.

Always start by removing leaves and large debris.

Then mechanically clean the bottom.

When significant organic deposits are present, Boost & Clean can facilitate deep cleaning.

Then support the biological balance with a solution designed for natural swimming pools, such as BactoSwim.

The goal is not to achieve a sterile pool.

The real goal is a living pool… in which biology works for you rather than against you.

Need to know which product or robot is suitable for your natural swimming pool?

Send us a few photos of your pool, its dimensions, depth, and the type of surface lining.

The Foudebassin.com experts can guide you toward the solution best suited to your installation.

→ See our solutions for maintaining natural swimming pools

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Frequently asked questions

Why are the walls of my natural swimming pool slippery?

A thin biological layer naturally develops on submerged surfaces. It can become more substantial when the pool contains large amounts of organic matter and nutrients.

Can biofilm be completely removed from a natural swimming pool?

It can be greatly reduced in the swimming area, particularly through brushing and vacuuming, but it will naturally reform. Some microorganism communities are also essential to the functioning of biological filtration zones.

Which robot should you choose for a natural swimming pool?

Choose a robot designed for swimming ponds and biopools. The AquaForte BioPool Robot Plus is notably equipped with brushes designed for natural surfaces and biofilm.

Can Boost & Clean be used in a natural swimming pool?

Yes. Aquipond Boost & Clean is also intended for natural swimming pools. Use it at 30 g/m³ with a water temperature above 15 °C. Always follow the product instructions and ensure that suitable circulation and filtration are in place.

What is BactoSwim used for?

Aquipond BactoSwim is a blend of natural bacteria intended for natural swimming pools and swimming ponds. It supports their biological balance and helps break down organic matter present in the system.

Robot or bacteria: which should you choose?

The two do not serve the same purpose. The robot physically removes the material present in the pool, while the bacteria support biological processes. In many installations, combining them as part of a consistent maintenance strategy is more relevant than choosing one or the other.

Additional technical source: “Natural Swimming Ponds as an Application of Treatment Wetlands — A Review”, Water, 2023.