My Backyard Aquaponics System: A Split-Flow System Explained

How I built my backyard aquaponics system using IBC grow beds, two fish tanks, different growing media and a whole lot of PVC pipe.

If you've ever wanted to grow your own food and raise fish at the same time, aquaponics is worth looking into. It's a way of combining aquaculture (raising fish and other aquatic animals) with hydroponics (growing plants without traditional garden soil) to create a system where the two work together.

I've been building my own backyard aquaponics setup here in the Top End of Australia, and I've designed it as a split-flow system. This gives me the flexibility to experiment with different growing media, growing methods and aquatic livestock, all while using the same central water system.

My setup uses two 1,000-litre IBC fish tanks, four IBC grow beds, a roughly 700-litre sump tank, a 10,000-litre-per-hour water pump and two radial flow settler filters.

It's a bit of a backyard science experiment, but that's half the fun!

In this guide, I'll explain how my system is put together, how the water moves around it, and why I've chosen to set up the different grow beds the way I have.

1. The sump tank: The heart of the system

Everything starts with my sump tank.

 

The sump is approximately 700 litres and acts as the central collection point for water returning from the grow beds and fish tanks. It is also where the main pump sits, ready to send water back around the system.

I'm using a 10,000 LPH pump to move water out of the sump and into a 40mm PVC pipe. This is the main supply line for the grow beds and fish tanks.

The main pipe runs up from the sump before reaching a 40mm-to-25mm PVC tee reducer. From there, 25mm PVC pipe branches off to supply 2 of my grow beds the water then continues to another 40mm-to-25mm PVC tee reducer and flows to my other 2 grow beds.

Each grow bed has its own valve, which allows me to adjust the amount of water entering that bed or shut the supply off completely.

This is an important part of the design. If I need to work on a grow bed, clean something out or change the growing media, I can isolate that bed without having to shut down the entire system.

After supplying the grow beds, the main water line continues towards the fish tanks. It splits through a 40mm PVC tee joiner, with each branch feeding one of the two IBC tanks. Each tank has its own valve, so I can control or isolate the water supply to either tank.

This is what makes the system flexible. Rather than having everything dependent on a single flow path, I can control the different sections individually.

2. The two IBC fish tanks

My system has two 1,000-litre IBC fish tanks, each with a slightly different mix of aquatic life.

The first tank houses three largeish barramundi, nine massive goldfish and a few red claw yabbies.

The second tank has a large population of rainbowfish, nearly 30 red claw yabbies and a couple of other fish I've caught in my scoop net.

Having two separate tanks gives me the opportunity to keep different species and experiment with raising fish and freshwater crustaceans alongside the plants in my garden.

The water supplied to both tanks comes from the main 40mm PVC line. As mentioned earlier, a tee splits the line into two branches, each with its own valve.

This means I can regulate or shut off the incoming water to either tank independently.

But getting clean water into the fish tanks is only half the job. We also need to deal with the water leaving them.

3. Getting water out of the fish tanks: SLO pipes

To move water out of my fish tanks, I use something called a solids lifting overflow (SLO).

A SLO helps draw water and suspended waste from lower down in the tank, where fish waste and uneaten food can accumulate. The water is then directed towards the filtration system.

My barramundi tank uses 50mm PVC for its SLO, while the red claw tank uses 40mm PVC.

I've also incorporated tee fittings into the SLO arrangements to provide an overflow path if the power goes out. The idea is that the tanks can maintain their water level rather than continuing to drain through the plumbing and potentially emptying themselves.

This is an important consideration when building an aquaponics system. Power failures happen, pumps stop working, and a system needs to be designed with those possibilities in mind.

The water leaving each fish tank then travels to its own radial flow settler filter.

4. Radial flow settlers: Removing the solids

The next stage of the system is filtration.

I'm using a 200-litre blue plastic barrel as a radial flow settler (RFS). Each fish tank has its own radial flow settler.

A radial flow settler is designed to slow the water down and allow heavier suspended solids, including fish waste and uneaten feed, to settle out instead of travelling straight back into the sump and then the rest of the system.

This helps reduce the amount of solid waste circulating through the plumbing and returning to the sump.

The water from each fish tank's SLO enters its respective filter, where the flow pattern encourages solids to settle towards the bottom of the barrel.

The water then exits the filters through 50mm PVC pipe and into 150mm PVC drainage pipe and travels downhill under gravity towards the sump tank.

Using gravity for this return journey helps keep the plumbing relatively simple. Once the water has made its way through the fish tanks and filtration stage, it returns to the sump, where the pump can send it around the system again.

The radial flow settlers are designed primarily for mechanical solids removal. They are not the only thing responsible for maintaining water quality: biological filtration and adequate aeration are also important parts of a healthy aquaponics system.

5. My four grow beds: Four different growing environments

One of the things I enjoy most about this system is that I'm not limited to one type of grow bed or growing media.

All four beds are made from the tops of IBC containers. Some were cut open by me, while others were already cut when I acquired them.

I've positioned the system along the side of my house, where the ground has roughly a 20% gradient. I've used that slope to help with the layout and gravity-fed drainage.

The four grow beds are divided into two different groups.

• Two flood-and-drain beds using LECA clay balls and Mt Bundy granite.

• Two constant-flow, dual-root-zone beds using sand and 38-litre fabric grow bags.

Each group operates a little differently, which allows me to experiment with different plants and growing techniques within the same system.

Grow bed one: LECA clay balls

LECA stands for Lightweight Expanded Clay Aggregate. These are the lightweight, expanded clay balls commonly used in hydroponics and other soilless growing systems.

The LECA bed is set up as a flood-and-drain system. Water enters the bed, the water level rises, and the bed then drains through a bell siphon.

I'm currently growing plenty of spring onions and sea purslane in this bed.

Sea purslane is particularly interesting because it's a salt-tolerant plant, known as a halophyte. It's a useful edible plant to experiment with, especially in a tropical garden where I'm always looking for plants that can handle our conditions. (See my blog about growing it!)

Grow bed two: Mt Bundy granite

The second flood-and-drain bed uses Mt Bundy granite from Sage Garden Supplies.

Rather than using lightweight clay balls, this bed uses granite as its growing media. I wanted to experiment with a different medium and see how the plants perform compared with those growing in the LECA bed.

At the moment, I've got Thai basil growing in this bed. It's struggling a little, although it's still growing reasonably well.

Because the beds share the same circulating system water, comparing them gives me an opportunity to investigate how the different media affect root conditions and plant growth.

Of course, the media itself isn't the only variable. Water flow, root-zone oxygen, nutrient availability and other growing conditions can all affect plant health.

Grow beds three and four: Sand and dual-root-zone growing

The other two beds are where things get a little different.

These are my dual-root-zone grow beds, and they're designed around 38-litre fabric grow bags.

Instead of using a traditional flood-and-drain arrangement, these beds operate with a constant flow of water.

The sand acts as a wicking medium, helping draw moisture upwards towards the plants' root zones. The fabric grow bags provide a separate growing environment for plants, allowing me to experiment with combining aquaponics water and a more conventional growing medium.

This approach gives me the opportunity to grow plants that might benefit from a different root environment compared with plants growing directly in LECA or granite.

The 38-litre fabric grow bags are also available through my store at Barry's Backyard.

The plant selection in these beds is a bit of a tropical mix. I've currently got around eight Thai bird's-eye chilli plants, mint, spring onions, sawtooth coriander, Thai basil, a pineapple, a large mulberry cutting and a couple of marigold flowers growing in them.

It's a good way to make use of the space and test how a range of edible and companion plants perform in the same system.

6. How the flood-and-drain beds work

Let's take a closer look at the two beds using LECA and Mt Bundy granite.

Flood-and-drain systems periodically fill a grow bed with water before allowing it to drain. Depending on the system design, this can help deliver water and nutrients to the roots while allowing the root zone to re-aerate during the drainage cycle.

In my setup, each of these beds uses a bell siphon to manage the drainage.

The standpipe and bulkhead

At the bottom of each bed, I've installed a 25mm bulkhead fitting. This creates a sealed connection through the bottom of the grow bed for the drainage pipework.

A 25mm standpipe sits inside the fitting. I've cut it slightly below the top of the grow bed so the water level doesn't rise all the way to the surface of the growing media.

This helps prevent the bed from overflowing while keeping the uppermost layer of media from becoming completely submerged.

The bell siphon

The bell itself is made from 65mm PVC pipe, with an end cap glued onto the top. I've cut openings into the bottom of the pipe so water can enter.

Around the bell, I've installed a 100mm PVC shroud with an end cap at the bottom. The shroud sits over the bulkhead area and protects the siphon assembly from the surrounding growing media.

I've screwed the end cap onto the PVC so that small, inquisitive hands can't easily pull it out. I've also cut slots into the shroud to allow water to pass through.

As water enters the bed, the water level rises. Once the conditions inside the bell siphon reach the point where the siphon starts, water is pulled through the drainage assembly and the bed begins to empty.

When the water level drops far enough to break the siphon, the drainage slows and stops. The bed then starts filling again as water continues to enter.

When everything is working properly, this creates a repeating flood-and-drain cycle without me having to manually turn the water on and off.

Bell siphons can be a little fiddly to get working consistently, but once they're operating correctly, they're a handy way to automate this type of grow bed.

Returning the water to the sump

Once water leaves each bell siphon, it travels through more 25mm PVC pipework to a 25mm elbow.

The elbow helps direct the flow into a 50mm PVC section, which then connects to the larger 150mm drainage pipe.

From there, gravity takes the water back to the sump tank, ready for another trip around the system.

The result is a grow bed that fills and drains automatically while the main pump continues supplying water.

7. How the constant-flow dual-root-zone beds work

The two sand-based beds operate differently.

Rather than repeatedly filling and emptying, water flows continuously into these beds. I've kept the 25mm PVC supply pipe and its valve, but reduced the flow further using 13mm poly pipe with another valve.

This extra control lets me slow the water down considerably. I don't want to send more water into the beds than their drainage can comfortably handle.

The sand helps wick moisture upwards, bringing water into the root zone where the plants can access it.

The fabric grow bags sit within this arrangement, providing a separate growing medium for the plants. The idea is to combine the advantages of a constantly supplied aquaponics system with the flexibility of growing in fabric pots.

Drainage for the dual-root-zone beds

The drainage assembly is similar to the one used in the flood-and-drain beds, but there is one important difference.

I've used the same 25mm bulkhead fittings and 100mm PVC shrouds with end caps. However, these beds don't have the standpipes used in the flood-and-drain beds.

This means some water remains at the bottom of the beds, even if I turn off the incoming water supply.

That retained water is part of the way I've configured these beds, but it also means I need to keep an eye on drainage and ensure the water level remains suitable for the roots.

The main goal is to provide a steady supply of water without flooding the grow beds.

8. Why use a split-flow system?

The main reason I like this setup is flexibility.

I can control the flow into each grow bed individually, isolate either fish tank's incoming water supply and experiment with different growing media without rebuilding the entire system.

The two flood-and-drain beds let me experiment with LECA and granite, while the two constant-flow beds allow me to try a sand-based, dual-root-zone approach using fabric grow bags.

All four beds are connected to the same overall system, but they're not all doing the same job in the same way.

This is useful when you're learning what works best for your plants and conditions. Rather than committing everything to one growing method, you can compare different approaches side by side.

The layout also makes maintenance easier. If I need to work on a grow bed, I can shut off its individual supply valve without automatically shutting down every other bed.

That said, isolating a grow bed doesn't mean every part of the system can be ignored. The fish tanks still need adequate water circulation, aeration and filtration, and any isolated section needs to be monitored to make sure it remains safe for the plants and aquatic animals.

9. Building an aquaponics system in the tropics.

Gardening in the Top End presents its own challenges. Heat, humidity, heavy seasonal rain and the conditions inside a tropical backyard all influence how I approach growing food.

One of the reasons I'm interested in aquaponics is the opportunity to combine food production with a system that lets me experiment and learn as I go.

I'm not claiming that my setup is the perfect design or that every plant will thrive in every growing medium. In fact, part of the reason I enjoy the system is being able to see what works, what doesn't and what I can improve.

The different grow beds are a practical way to compare growing methods using the same circulating water. If one bed performs differently from another, I can investigate the possible reasons rather than relying entirely on guesswork.

Water chemistry, nutrient levels, root health, oxygen availability, water flow and growing media all deserve attention. A healthy aquaponics system is about balancing the needs of the plants and the aquatic animals, not just getting water to circulate.

Final thoughts: Grow your own damn food

My backyard aquaponics system is an ongoing project. There's always something to adjust, something new to plant and another idea I want to test.

At the moment, I've got four grow beds using three different growing media, two fish tanks, radial flow filtration and a sump that brings everything together.

The combination gives me plenty of room to experiment with edible tropical plants, fish, red claw yabbies and different ways of growing food in a backyard.

I'll keep documenting the system as it develops, including the successes, the problems and the lessons learned along the way.

If you're interested in building your own backyard food-producing setup, follow along with Barry's Backyard. My goal is to help more people grow their own food, especially here in tropical Australia, where our climate calls for a slightly different approach to gardening.

Grow your own damn food.

Want to try the dual-root-zone method? Check out the 38-litre fabric grow bags available at barrysbackyard.com.au.