Converting a Swimming Pool to Grow Fish

Silver Perch in this swimming pool are now weaned off fish pellets

One of the advantages of doing aquaponics is that it gives you the knowledge of how a natural system works.

For Les and Annette Mulder it was only a natural progression to apply their knowledge of fish farming to their 55,000 liter chlorinated swimming pool located in Sydney.

Les switched off the swimming pool pump over a year ago and decided to grow one hundred silver perch the same way a natural system occurs in nature. This meant the fish would have to survive on their own.

Could such a thing work in a suburban sized swimming pool?

The decision to stop putting chlorine into your pool and allow nature to take over can take a bit of courage – especially if your friends think you are mad.

“Our friends think we are green crazy weirdo nuts!” said Les.

Annette laughed, “The pool got darker and murkier and started looking like something from the black lagoon!”

“It really was black and disgusting as the chlorine came out but it was still too toxic for anything to live in there other than some algae and things like that.” said Les.

“Eventually some Mosquito larvae began to turn up in the water and that meant we could throw some Pacific Blue-Eyes (a small variety of fish) in there.”

For filtration Les arranged three bathtubs in a row and filled them with gravel. A small 60 Watt pond pump will be used to recirculate the water through the bathtubs and that water would return filtered clean back to the pool. The growbeds were stocked with aquatic plants such as Taro, Papyrus, Louisiana Swamp Iris, Chinese Water Chestnuts and Mints. Les and Annette say the couple wanted to choose plants that were very good at filtering water.

“The aim is to try and get a system that was in balance faster than the aquaponics system.” said Les.

This 55,000 Litre Swimming Pool is converted to a small fish farm using very little energy

“We want to get to the point where we got with our other aquaponics system, where it becomes self cleaning and self regulating…” says Les. Planting aquatic plants was a way to fast track that conversion.
Les has also placed a large children’s clam shell swimming pool underwater at the right height that is also filled with sand that grows water chestnuts.

All these additional aquatic plants have had an unexpected benefit.

Various microscopic animals eventually moved into this swimming pool system and the fish are thriving living off small insects, mosquito larvae, algae and fresh shoots from plant roots.

The fish were living in a natural system. We were flabbergasted when Les said he does not feed the fish at all. No fish pellets from depleted reserves of ocean catch are needed. According to Les the fish at five months of age were doing extremely well finding their own food. He believes they are growing faster than if they were fed on commercial fish pellets. The system is looking after itself.

The other thing the couple noticed were the re-emergence of rare frogs that came to live in their pool system.

“We even got a species that we never had around her before.” said Les. The emerald spotted tree frog began croaking from the gravel bathtubs.


We were filming at this pool with Permaculture teacher Geoff Lawton who agreed that most people spend a lot of money actively trying to stop algae from appearing in their pools, yet here was a system where algae was encouraged to grow and help feed the fish. There is some evidence to support the theory that some algae are also very high in Omega-3 fatty acids a health benefit to people that will eventually eat the fish. The added benefit as Les also discovered is the drop in electricty costs from switching off his main pool pump. Rather than 1 kilowatt of energy spent running a conventional chlorinated pool per hour, he had reduced his costs to a fraction. Because of the large surface area of the pool, little aeration was also needed. A small trickle pvc pipe returns a steady stream of water back to the pool, which provides the only air back into the system. Before you run off and build your own system, be mindful that this system requires a large body of water (50,000 litres) to become effective.

One hundred Silver Perch in such a system are at a very low stocking density when compared to conventional aquaponics systems. But the advantages are great. Mechanical failure of pumps means the fish will survive if you forget to turn the power on.  Fish will find their own food. The nutrients that the fish provide will also be on the low side so three bath tubs of filtration may be all the food you can grow unless you decide to add more fish. But for a leisurely approach to taking it slow, easy and letting nature do most of the work – getting 30 to 50 kilos of fish at the end of the year seems like a real bonus for very little inputs.

This system will be featured in The Urban Permaculture DVD with Geoff Lawton later in the year.