How does the water stay clean?

Clear, clean water is the naturally occurring state in ponds and lakes if the body of water is large and deep enough. Swimming ponds are purposely built to emulate this state and are a prime example of how we can harness nature's intelligence effectively. 

A conventional swimming pool uses chemicals to kill micro-organisms in the water such as phytoplankon (microscopic single-celled algae, which in abundance make the water go green) and bacteria. However this isn’t necessary in a natural pool where the cleaning work is carried out by micro-organisms which exist together in equilibrium. 

This is the basic principle of a swimming pond - to use the natural purifying properties of plants and micro-organisms to sustain clean, clear and healthy water.
 

So you don't have a filter then?

In natural water, the main filter is the whole water body, as each part of water is constantly being filtered by microscopic life-forms. 

However, in swimming ponds it is also essential to keep the nutrient levels very low, to ensure that single-celled algae is always controlled by zooplankton, and to prevent the growth of blanket weed (string algae). To do this we filter the water through shingle and other mediums using a small pump.  Plants also perform a useful function taking nutrients out of the water as they grow.  By keeping the levels of key nutrients low and in balance, algae is not able to grow and the result is clear, clean and healthy swimming water.

We use standards required by the German swimming pond association (more stringent than the International swimming pond standards, which are adopted by default by the British Association of Natural Swimming Pools), which are used in hundreds of public swimming ponds in Germany and thousands of priviate swimming ponds.  

Below is a very simplified drawing of how our filter system works - in reality there are many elements which go into making up an effective filtration system but this shows the basic principle:
 

"People worry about pond water but I would tell them how clean and clear it is, and how relaxing."

-- Annie Noble, Bristol

"The water is wonderfully clear and you find yourself drawn to the space. You want to just sit out there with a cup of tea, absorb the peace and contemplate the world."

-- Sean Moore, East Sussex