Converting a Swimming Pool to a Koi Pond

Follow the successful conversion of a 100,000 litre swimming pool to a koi pond.


Converting a Swimming Pool to a Koi Pond

By Jack Birkenbach

In August 2010, Jack Birkenbach received a call from a friend who had acquired a new property. He intended to expand his current business as well as develop an office block with conference facilities in the existing home. There was a large swimming pool on the property and this was deemed inappropriate for a business environment. So Jack was approached to consult on design and do the actual conversion to a make it into a koi pond. One of the complications was the rezoning of the property for business purposes by the authorities and the option to revert back to a swimming pool had to be built into the design.



In the background, the house, designated Offices and the swimming pool looking towards the deep end.

Current offices, created from previous 'Entertainment Area' to the right of the pool.


Jack enjoys a challenge when it comes to designs and actually doing the job. The whole project was carefully planned on paper and various options in terms of filtration were considered.

The pool construction is Concrete with a marbelite finish which had been painted with blue epoxy paint. Size is 15 x 5 metres, with an average depth of 1,4 metres. Volume is an estimated 100,000 litres

Jack did not want to damage the surrounding paving or basic structure so that the pond could be converted back to a swimming pool.

As the pool had been without water for some time, it was decided to complete the shell first and fill with water before commencing with the construction of the trickle tower/filter system. This was a demanding task, very labour intensive, and several flapper grinding discs were used. The existing pool skimmer was to be used and  incorporated into the final design. The first task was to strip the blue epoxy pool paint and prepare the underlying surface for the sealant. Minor cracks on the floor were opened and sealed with a product called Hydrostop. The sealing of the pool was done with two coats of Hydroseal.

In the interim, before the filtrations system was constructed the water circulation would be through the original weir / skimmer.




Preparing the Surface

Sealing cracks


The filtration system presented a major concern as the integrity of the swimming pool had to be preserved but still have a sufficiently large system for the volume and number of koi that would be housed. While the shell preparation continued a member of the SAKKS committee was approached for expert advice. After a few visits, and much correspondence, the advice was to build a trickle tower system, preceded by a large mechanical filter. This option was decided on, and building commenced. The Trickle Tower/Filter system, because of its size, was positioned behind a screen wall.


The filter chamber concealed behind the foliage.


The existing surface skimmer was used. New larger diameter piping from the surface skimmer to the pump and on the existing returns was used.


A two chamber design was selected. The first chamber would be the mechanical filtration and the second chamber would be a trickle filter. Plastic media was used in both chambers. The mechanical filter was to be and up-flow system and the trickle filter was obviously a down flow system. Incorporating them into one filter was a clever design feature.

Foundation excavation 400 wide x 400 deep. Reinforced steel boxes placed in excavation prior to pouring concrete.



Filter Chamber foundation preparation


Filter Chamber build



The internal dimensions of the complete filtration chamber would be 3100 x 1700 x 2300 (mechanical filtration 2400 x 1700 x 2300, and Trickle Tower 1500 x 1700 x 1800.

Ref. 193 welded mesh was placed in the concrete base, which was cast 220mm thick. Every 900mm Y10 (10mm) rebar was vertically positioned - and embedded - in the base, and connected to the mesh. This would serve to reinforce the structure between the two rows of bricks. Between each layer of bricks a course of SABS brickforce was placed.

Each chamber (Mechanical and Trickle Tower) had a 110mm bottom drain to do periodic maintenance by completely draining the chambers and washing the media down to waste. The chamber walls were plastered and sealed with Hydroseal.

Each chamber had a grid built at the bottom from plastic pallets and simply reinforced with PVC piping and cable-tied together. This grid was raised off the floor to create an even hydraulic load on the bottom of the filter media in the up-flow mechanical filter and an open chamber at the bottom of the trickle filter part.


Plastic grid under construction to tensure even hydraulic load. Plastic palleys were used for the floor of the grid.



Water from the pond is drawn off the bottom by a 1,1 kW Speck Galaxy pump, through 75mm PVC pipe with numerous 12mm holes drilled into it. The 75mm pipe is reduced to 63mm at the pump inlet, and increases to 75mm immediately after the pump. This is fed to the bottom section of the mechanical filter chamber. The water rises through seven layers of bagged plastic shavings, overflows through nine 75mm pipes (drilled with 6mm holes), to create the shower for the Trickle Tower.



Finished Chambers with piping between the chambers to ensure even water movement.




The mechanical filter has an air bump system at the bottom to assist with cleaning the media. The pump is shut off. The air pump is turned on and the whole mechanical filter chamber bubbles like a kettle. This air bumping dislodges much of the organics and solids trapped in the media. The bottom drain is opened and the entire chamber is drained. Water is then poured of the media from the top to rinse it. The mechanical filter is working very well at keeping the water clear and the regular flushing is removing the solids easily.



The second chamber in the filtration system is the trickle filter. The upward flow of water from the mechanical filter is moved between the chambers via nine 75 mm pipes with a drilled pattern of 6mm holes. This creates the spray bars for the Trickle Tower. Thirty two 50 litre crates filled with plastic shavings provide the media for the biological filter. Four layers of eight crates each provide 1600 litres of filter material for the Trickle Tower.




The trickle tower in operation



Every hour, thirty two thousand litres pass through the Trickle Tower.



There are 4 x 110mm pipes returning the water from the bottom of the trickle filter to the pond. These are angled to generate circulation. The water pouring down through the Trickle Filter oxygenates the water and nitrifying bacteria there removes the ammonia, nitrites and nitrates.

The returns into the pond also add oxygen as well as circulating the oxygen rich and purified water throughout the pond.



The swimming pool weir was used as the surface skimmer for the pond. The 50mm PVC pipe from the weir was replaced by a 63mm PVC pipe leading to a 1,1 kW Speck Galaxy pump. From the pump water is returned to the pond via two 4 bag Collins Sandfilters connected in parallel, each followed by a 55 W UV light. There are two returns to the pond from each sandfilter which also generate circulation within the pond.







The first phase of the conversion has been completed, and some koi have been introduced. The returns to the pond from both filter systems still need to be concealed with landscaping and appropriate gardening and cascades. But, as mentioned in the introduction, this phase will only be concluded once the application for rezoning of the property for business purposes has been approved by the Tshwane Metro.



The finished swimming pool to koi pond conversion.


Some very happy koi and growing extremely well. Pond still going through the new pond syndrome.