How aquaponics works

The loop, step by step

1. Fish are fed and produce waste. Fish excrete ammonia through their gills and in their waste. In a closed tank, ammonia builds up and becomes harmful to the fish.
2. Bacteria convert ammonia into nitrates. Two groups of naturally occurring bacteria live on the surfaces inside the system. The first converts ammonia into nitrite, and the second converts nitrite into nitrate. This process is called nitrification, and it is the engine of every aquaponics system.
3. Plants absorb the nitrates. Nitrate is a nutrient plants take up readily. Growing plants draw it, and other nutrients, out of the water.
4. Clean water returns to the fish. With the nutrients removed by the plants, the water flows back to the fish tank and the cycle starts again.

Water is lost only through evaporation and through what the plants take up, so the system is topped up rather than drained.

The parts of a system

• Fish tank — where the fish live. Its size sets the capacity of the whole system.
• Solids filter — removes solid waste before it clogs the grow beds.
• Biofilter — surfaces where the nitrifying bacteria live. In media-bed systems, the gravel or clay media does this job.
• Grow beds — where the plants grow (see the three methods below).
• Pump and aeration — move water around the loop and keep oxygen levels up for fish, bacteria and roots.
• Sump — a low collection tank that makes pumping and topping up simpler (used in some designs).

Three ways to grow the plants

• Media beds — plants grow in beds of gravel or expanded clay that also act as the filter. Robust and simple. Good for homes, schools and fruiting crops.
• Deep water culture (rafts) — plants float on rafts, with their roots hanging in the water. Well suited to leafy greens at commercial scale.
• Nutrient film technique (NFT) — a thin film of water runs through channels past the roots. Space-efficient for light, fast-growing crops.

What grows well in Uganda

Fish. Tilapia and African catfish are both widely farmed in Uganda and tolerate aquaponics conditions well.

Crops. Leafy greens (sukuma wiki, lettuce, spinach, amaranth/dodo), herbs (basil, coriander, mint), and fruiting crops such as tomatoes and peppers in mature systems.

Looking after a system

• Daily: feed the fish, check that the pump and aeration are running, look at the fish and plants.
• Weekly: test the water (pH, temperature, ammonia, nitrite, nitrate) and top up.
• As needed: harvest, replant, clean the solids filter.

The trade-offs to know

• It needs power. Pumps and aeration run every day, so plan for power cuts, with a solar or backup option for aeration.
• It costs more to start than a garden. Tanks, pumps and plumbing mean a higher upfront cost, which is recovered over time through produce and fish.
• There's a learning curve. Water chemistry and fish care take a few weeks to learn, which is why our Build & Train package includes training.
• It's a living system. Fish, bacteria and plants all need attention, and a new system takes several weeks to establish its bacteria before full stocking.