The first thing we do in the morning is to check the pH in the subtank. The pH, a measure of how acidic the water is, determines the health of the fish, the creation of bacteria in the system, and the ability of plants to absorb nutrients. The pH range for freshwater African catfish is between 5.0 and 7.0 – slightly acidic. If the pH is higher or lower it is harmful or even deadly for the fish. For that reason we try to maintain the pH between 6.0 and maximum 6.5. By checking the pH we can see which nutrients need to be added. Potassium sulphate is added to decrease the pH and potassium hydroxide to increase it. Other elements included are bone marrow, iron, and Epsom salt in addition to nitrogen that comes from the fish tank water, enriched with ammonia by the fish excretions.
Everyday the kitchen of our restaurant needs fresh herbs from the aquaponic greenhouse depending on the different menus. An additional task is to find out which plants grow best in the aquaponic.
Typical plants we harvest daily are varieties of mint (Moroccan mint, apple mint, peppermint), Vietnamese coriander, pineapple sage, edible flowers like begonia and malva, Szechuan flowers and Indian cress.
After lunch we feed the fish around 14:00 every day. We note down how the fish react to the food as it is an indication of the health and living conditions. The amount of fish food that we give to the fish depends on the fish weight and on the season, in summer we feed them more.
The rest of the tasks can vary every single day from sowing new plants according to the season, trimming, maintenance work (e.g. cleaning pipes) and researching how we can optimise the aquaponic system in a more efficient way.
Weekly, we do a water test to measure the levels of nitrates, nitrites, ammonia, iron, calcium, magnesium, potassium and phosphate. This way we can see if we are still adding the right amount of nutrients. Monthly, we send one water sample to a lab to check our values against the lab results. We finish the day around 18:00 by cleaning and tidying the greenhouse.
What is an Aquaponics system?
Aquaponics is a system that combines aquaculture and hydroculture by breeding fish along with plants. It is a semi-closed circular system whereby water is recycled with a constant water flow that connects the fish tanks and the grow beds, whereas fish food and certain nutrients are added every day. The fish excretions enrich the water with ammonia which is converted to nitrite and then to nitrate by two types of bacteria. Nitrate is the available form of nitrogen for plant uptake, one of the essential macro-nutrient for plants. This process reduces the ammonia levels in the water to a safe level for fish, making it possible to circulate back the water into the fish tanks. The system maximises the use of nutrients, water and energy. Regular checks and maintenance works are needed to preserve suitable environmental conditions for both fish and plants.