Aquaponics Part 5: From biology to hardware
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ast month we focussed on the bacteria system as the crucial element in aquaponics to form a complete ecosystem with the fish and plants. It is very important for an aquaponics system to remain in balance to create optimal conditions for health and growth. The system is in balance when the ammonia released by the fish is changed into nitrites and nitrates by bacteria, and when there are enough plants to absorb and use up the nitrates before the water goes back to the fish. This happens when the number of fish and plants are in balance and the bacteria had enough time to do their work. This process is called: Cycling up. When starting a new system, this process can take weeks, depending on the size of the system. In the chart below this balance or “sweet spot” is reached where the blue and green lines cross:
Figure 1: pdfs.semanticscholar.org 8
The question most new aquaponic growers struggle with is: How many fish do I need for how many plants and how big must my fish tank and grow area be? Unfortunately, there is no easy formula – because fish and plants both grow. Bigger fish release more ammonia and give more nitrates. Smaller plants need less nitrates. You can’t only use the number of fish or plants as a basis; it is more accurate to use the quantity of fish food needed as the starting point of your calculations. Inevitably you give less food to smaller fish. Tilapia eat about 1% of their body weight per day. In other words, a 100 g fish eats 1 g fish food and a 1 kg fish eats 10 g fish food daily. You can determine the amount of fish food needed by weighing 10 fish and calculating the average weight. The formula determining the balance between fish and plants was worked out by Dr James Rakocy of the University of the Virgin Islands, who developed the well-known UVI aquaponic system. It is the first commercial system based on scientific studies. The formula was later refined by a colleague of him, Dr Will Lennard. The formula we use is: 20 g/m2/day. That means you need to feed 20 g of fish food per day to enable 1 m2 of growth in a raft system.
The guidelines are: • Decide how many plants you want to grow – for example 30 heads of lettuce (1 m²). • If you want to plant 30 lettuce plants, you need 20 g of fish food – that means you need 2 kg of fish. 20 g fish food
2 kg fish
1 m2 plants
ProAgri BNZ 05
