A peek into the fish tank - aquaponics explained

It has been well over 6 months now since the hydroponics system was first set up in the Permapatch Community Garden. And what a success it has been! Rows and rows of fresh crunchy lettuces in all shapes and colours.

And - despite us knowing that growing strawberries in Sydney (and a garden full of magpies) would always be a challenge - we even harvested a small crop of strawberries. 

Shannon and Anthony are our aquaponics specialists and together with Trevor from HydroMasta, Colin, Roman, and many other helpers in the garden they have put hours of sweat, love, and quite possibly a few tears into the beautiful system.  What started out as hydroponic (when we added nutrients manually) turned aquaponic earlier this year. In March, fish were introduced into the system and their waste now provides the nutrients. 

Asked what kind of fish we have, Shannon explains that we have silver perch in the system. Silver perch were selected because they can handle some temperature variation and high nutrient conditions. We have been running the system with only 100 fish to start, to make sure that everything works. It will take approximately 12 month for the fish to reach plate size and be ready for eating. As they are predatory fish, we can't add any more to the system as they will eat the younger fish. We will monitor the nutrient loads in the system while the fish are growing and determine how many fish we think the system can ultimately handle. 

Shannon adds that silver perch are highly adaptable fish that don't require sunlight, so they will feel comfortable in the tank. The fish are fed to provide them with the main nutrients they need. They also eat algae, which will grow within the tank. We've also included a couple of oxygen stones in the tank, to keep dissolved oxygen levels high. 

So how exactly does it all work? There are two main components - a fish tank and a plant bed. Nutrient-rich water from the fish tank is pumped through a bed where plants take up the nutrients to grow.

When we feed the fish they excrete waste, either in form of solids or solubles. The solubles can eventually be taken up by plants, however we don't want the solids to accumulate. Therefore a clarifier has been designed to create a centrifugal swirl. It drives the solid waste to the bottom of the blue tank. Take a look at the diagram below - in the middle you have the green fishtank and the blue clarifier. The solids settle down in it and can be removed by opening the tap at the bottom. The clear water above the solids is returned from the clarifier to the fish tank.

Let's follow the nutrient-rich water as it is pumped to the plant bed. On its journey, ammonia is dissolved in the water. However, in that form the nitrogen is not available to plants.  Bacteria help convert it first to nitrite, then to nitrate, which can be absorbed by the roots. With nitrate removed, clean and healthy water is circled back to the fish to live in.   

The principle is fairly straightforward, however getting it all up running and ensuring it remains in balance for happy fish and happy plants is no easy feat: every morning and evening our tireless helpers on the chicken roster also feed the fish - a big thank you to everybody involved! 

We all can also help ensure everything is running as it should be. The easiest thing to check is that the pump is running - it needs to be on always. If you notice for example that the water is no longer flowing, that there is a leak or anything else a bit "fishy", please notify the aquaponics crew immediately: Colin on 0411 578178.

Of course, the happiness of fish and plants is regularly checked. In addition to everything else, Colin is monitoring nutrient levels. It's all  a question of balance. For example, in winter when plants grow more slowly and also fish activity decreases, we can halve the amount of food given to the fish. Growth and therefore feeding will pick up again, come spring.  

A big thank you to Shannon and Anthony for providing this information. We look forward to many more lettuce harvests.