Build your own soil-free aquaponics garden for leafy greens, chillies, herbs and more

Four years ago, while living in a tiny, inner-city Adelaide apartment, Alexis Branlard decided to experiment with an unconventional approach to growing food in a rental.

After getting an okay of sorts from his landlord — "just don't go crazy," she said — Alexis built his own small and moveable aquaponics system, complete with eight goldfish and a thriving soil-free veggie garden.

The fish are fed on black soldier flies that Alexis and his partner Marion Vigot breed in their compost during summer, plus "a DIY diet we make using food waste".

Nutrients from the fish waste, in turn, feed plants growing hydroponically above, creating a self-sustaining system that pumps out food — currently, that includes chives, salad greens, spring onions, chilli, capsicum and strawberries.

The couple believes the system is perfect for apartment living, balconies and small-space renting, as it's completely independent from a garden or soil, and small enough to move with its owners.

Here's how they built their closed-loop aquaponics system.

Setting up the aquaponics frame

Alexis first went down a YouTube rabbit hole to learn as much as he could about the principles of aquaponics.

"We knew nothing about it at the beginning," he says.

He then designed a small system — not much bigger than an average cupboard — ensuring the couple could easily take it with them when they moved houses (which they've since done twice).

First up, Alexis created two, 110cm by 60cm rectangular boxes by joining four lengths of recycled plywood together with timber screws. These had a strong brace underneath capable of holding the weight of either water or plants. He added plastic pond liner to both boxes to make them watertight.

Next, he built a frame connecting the two boxes, plus a second frame for the top of the system, which unclips easily so the two elements can easily be pulled apart when moving.

Finally, Alexis cut three, one-metre lengths of recycled PVC pipes, using a saw kit to drill holes every 15cm for the plants to live within. He joined these into S-bend shapes using PVC connectors, and added aquaponics plant baskets to each hole.

How to create a self-sustaining water siphoning system

With the basic structure built and lined, Alexis filled the bottom wooden box with water to create the fish tank. He then filled the top wooden box with baked clay pebbles, creating soil-free veggie beds.

The final step was probably the trickiest: creating a siphoning system that cycles water through the whole structure, using just one strategically placed pump.

"It didn't work the first try — there were a couple of iterations," Alexis recalls.

In the end, after watching a heap more YouTube videos, he created the following circular system:

1. 1.A piece of poly pipe placed at the bottom of the fish tank. The weight of the water sitting in the fish tank slowly pushes water through the pipe into a bucket sitting outside.
2. 2.Inside the bucket are bio balls — essentially small plastic balls that allow water-cleaning bacteria to grow on them — which naturally filter out the solid fish waste.
3. 3.Another piece of poly pipe connects this bucket to a second bucket, also containing bio balls for a second round of filtration.
4. 4.A water pump sits inside this second bucket, to pump the now-cleaned water up to the very top of the aquaponics system and into the PVC pipe.
5. 5.The water runs through the PVC pipe S-bends and into the wooden veggie box below, allowing all plants to soak up any remaining fish waste nutrients as they grow and thrive.
6. 6.Inside the wooden box is an automatic bell siphon — a device that leverages the forces of pressure and gravity to empty the water from the veggie bed once it reaches a certain level — dumping it, now clean, into the fish tank below.

Aside from that one powered pump, the whole system runs itself automatically, simply using the pressure and weight of the water.

"It's almost like the tide table — the bell siphon mimics the sea going in and out," Marion explains.

Feeding the fish naturally, for free

For the first few years, Alexis and Marion made their own fish food by blending food scraps with oats and freezing them into cubes that could be fed to their goldfish, one by one.

Recently, they discovered black soldier flies breeding within their compost. So, they now supplement the veggie cubes with larvae shells or even live larvae.

"We decided to make our own fish food because we didn't want to give them bought fish flakes — who knows what that's made of and we're growing food with it, so we don't really want those weird ingredients to be transferred into our food," Marion says.

"The black soldier flies are a great source of protein. And there's nothing more wholesome than feeding your fish with food that you produce yourself — free."

Choosing veggies that thrive in aquaponic systems

The couple now eat almost daily from their aquaponics system. 

They've had huge success growing all types of leafy greens, herbs, tomatoes, chillies and strawberries — all of which withstand Adelaide's 40+-degree summers without any shading, thanks to constant water access.

But Alexis cautions that most root veggies are a no-go in aquaponics, as they can't survive the constantly wet conditions.

"Our next project is to add fish we can eat. Then we'll have delicious fish, organically grown in the backyard of our rental."

Koren Helbig practices permaculture and grows organic food in the backyard of her small urban Tarntanya/Adelaide home.