Want to step up your food growing to a new level? Consider aquaponics, a closed-loop system that can supply you with vegies and edible fish

Aquaponics combines fish and plants in a single production system that moves water continuously between the two environments via pipes, pumps and gravity.

Aquaponics is by no means new; the Aztecs used islands known as chinampas to undertake aquaponics, while the Chinese employed polycultural farming systems for thousands of years to produce carp and edible snails in rice paddies.

A modern aquaponic system has the potential to produce crops up to seven times faster with less than 10 per cent of the water and nutrients required in a terrestrial cropping system.

The aim is to grow produce using fish waste for nutrients, including green vegies like lettuce, spinach and celery, as well as fruit such as strawberries. Harvesting the fish for food is optional.

Whether you operate a large commercial aquaponic system or desktop aquarium that incorporates aquaponics, the principles are the same, but remember, the larger the system (500L or more), the greater the production and stability.

You can create your own set-up for less than $100 or buy a ready-made system for as little as $300.

How does it work?

The water in an aquaponics system is naturally re-oxygenated via the dispersal of bubbles, which provides oxygen to fish and roots of plants and supports beneficial bacterial microbes (Nitrosomonas and Nitrobacter). The microbes convert the fish waste (ammonia) to nutrients. Mechanical aerators can be added to increase the level of oxygen.

Healthy oxygen levels (5mg/L) are maintained by appropriate plant-to-fish ratios and the movement of water, as well as a routine of changing at least 10 per cent of the water in your system each week.

Keeping your system healthy

It’s important to regularly test the water quality so you can anticipate and prevent chemical imbalances that may impact on fish and plant health. A water quality test kit costs around $50. It tests for:

Ammonium: This is a by-product of fish waste and can become toxic and therefore is measured at weekly or daily intervals, depending on the intensity of your production and the time of year.

pH: A pH test measures hydrogen ions in the water and is usually required weekly. The pH range is 1–14 but a healthy range is fairly neutral, typically 6.5–7.5, depending on fish species. The nitrification process, which converts fish waste into nitrogen, will acidify the water, so monitor pH closely.

Water hardness: This measures the concentration of minerals in the water. The harder the water, the greater the concentration of minerals. Acceptable hardness levels again depend on the fish species you choose.

Tip: Higher temperatures accentuate oxygen loss and raise the rate of toxicity. Hence, you’ll need to test all aspects more often in warm weather, especially ammonia and pH, and ensure all pumps are functional and your fish stocking rate is not excessive.

By and large, all components of a system (other than air, some water and food for the fish) are recycled. However, since this is an artificial environment, a pump and filter must be used to circulate water and maintain its quality.

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Water circulation

Effective aeration should be achieved if the volume of water in your system circulates every hour, although the greater the circulation, the more fish and plants you can accommodate.

Some fish require higher levels of oxygen than others. The robust nature of carp makes them a good choice for the beginner, though perhaps not the best to eat (yes, I have tried them!).

The rate of water movement and effectiveness of filtration affect the rate of waste conversion by microbes in the system, so go for stronger rather than weaker circulation.

Tip: Solar pumps are an option but require batteries to operate 24/7 and are therefore less desirable to beginners.

Filtration

Physical: The main purpose of filtration is to remove solid particles from the system, maintaining plant and fish health. Solid wastes are tangible particles that must be removed to avoid decreasing oxygen availability. Solids may also adhere to plant roots, reducing the plants’ ability to uptake oxygen and nutrients.

Research the filtration options thoroughly if making your own system. Ready-made systems should already address filtration and the “settling out” of solid waste.

Biofiltration: This is the use of natural agents such as bacteria to detoxify waste. A bio-filter houses the nitrifying bacteria. Research biofiltration in depth if you intend creating your own aquaponics system. If purchasing a system, enquire about its inclusion.

Fish

Do your homework before deciding on fish species as requirements vary. Feeding frequencies vary according to the breed and age of the fish. Mature fish require less volume of food per kilo than juveniles. Feeding twice a day is a good guide.

If you don’t feed your fish, they don’t grow! Hence, if your plan is to eat them, be aware of daily intake requirements of various species. Some species are more efficient at converting protein to fat — catfish, barramundi and carp, for example, are better than perch in developing size with less food.

Tip: You can source fish from your local aquarium supplier, but online aquaculture sellers may offer a better deal, delivered to your door.

Plants

You can grow edible or ornamental plants in a system — it’s your choice. Leafy greens like spinach, celery and lettuce do best, but even strawberries and tomatoes do well with some supplementary feeding.

The plant grow beds can be any configuration you want, but are usually rectangular in shape and around 20–40cm in depth. Regular soil cannot be used in aquaponic systems; rather, products such as clay balls or natural stone are used as the growing medium. The advantage of clay balls is they are porous, fungus-free, light in weight, don’t rot and are hygienic, therefore perfect for microbial communities to inhabit and ideal for plant roots to explore.

A disadvantage is that clay balls are prone to blowing away and are relatively expensive. It will cost around $30 for
a 30L bag of expanded clay balls. The average grow beds might require 50–100L, hence $100.

Alternatively, use 20mm gravel or round stone, as long as you have a sturdy grow bed frame to accept the extra weight.

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Plant nutrition and protection

Between 13 and 16 essential nutrients are required by plants. Fish food typically contains 10 of the essential nutrients, missing calcium, potassium and iron. An aquaponic-specific fertiliser replaces the missing nutrients; otherwise, use calcium hydroxide and potassium hydroxide, along with iron in a chelated form. I have found celery and rocket grow happily without supplementary nutrients.

Plants in aquaponic systems need four to six hours of natural light a day or longer if artificial lights are used. However, fish prefer a subdued light, so most systems house the fish underneath the grow beds.

Tip: Apply the same organic insect control as you do in the garden. Use nets as an exclusion method and organic sprays only when necessary.

Types of systems

There are various options for growing plants in aquaponics systems.

Media-filled beds: The grow beds are filled with stone or expanded clay media and the system continuously pumps water through the beds or optionally uses a bell siphon to flood and drain the system every 20 minutes. The bell siphon increases oxygen and moves waste more effectively.
Nutrient film technique: In this system water is pumped across a network of pipes. Plants are placed in cut-away holes in the tops of the pipes and plant roots access the nutrients from the water below.

Deep water culture: In this system plants float in rafts and the roots remain in contact with water to extract nutrient, similar to the Aztec system. So, go on, explore the possibilities of building your own aquaponics system or purchase one to supply fresh greens, strawberries, tomatoes and fish for you and the neighbourhood!