Growing duckweed in your own tank is one solution during the warmer summer weather. Its not an algae but a very small aquatic plant. Because the water in an aquaponics system is rich in nutrients, it is well suited to also grow duckweed which is 30% to 40% rich in protein providing the water is undisturbed and slightly shaded. Many fish will readily eat this tiny plant that doubles in size rapidly and is an excellent supplement to feeding your fish, but you will need heaps of it to keep your fish alive.
Recently while filming The Urban Permaculture DVD with Geoff Lawton, we came across a family who were growing around 100 silver perch fish in their family swimming pool. They said they don't feed their fish any fish pellets at all. So how did the fish survive? By eating the insect larvae and tiny crustaceans that grew in the pool but mainly – the algae that naturally formed on the sides of the pool.
There are many varieties of algae but one of the champion varieties, Spirulina is one that has been written about a great deal. Mainly because it is extremely high in protein (60% – 70%) and has many other nutritional benefits as well. It is said to be rich in vitamins, minerals, and carotenoids (a type of antioxidant that can help protect cells from damage). Its full of all sorts of goodies, B complex vitamins, beta-carotene, vitamin E, manganese, zinc, copper, iron, selenium…in fact the list goes on and on.
有許多的藻類品種，但首選是螺旋藻。主因是為它的高蛋白（60％ - 70％），以及許多其他的營養源,如豐富的維生素，礦物質，和胡蘿蔔素。它富含複合維生素 B，β-胡蘿蔔素，維生素 E，錳鋅銅鐵硒等。
Test tube and animal studies suggest spirulina may boost the immune system, help protect against allergic reactions, and have antiviral and anticancer properties. So many companies are now marketing Spirulina tablets in health food shops around the planet.
Recently a new documentary on Spirulina is being released advocating its super food status. If it's good enough for NASA astronauts to eat in space – then it might be good enough for your fish to eat as well?
Growing Spirulina 養藻
So how do you grow Spirulina as a viable source of fish food? Can you grow it in open tanks attached to your aquaponics system? Well you can grow it in a small heated aquarium but it will be difficult if you live in a cool climate zone.
Spirulina only grows if the water temperature warm. The optimal conditions are a warm 35 degrees Celsius (95 degrees Fahrenheit). It will not grow in most back-yards over Winter where the temperature is well below that range. Below 20C it stops growing entirely.
The other problem is the pH of the water. It needs very alkaline water at around a pH of 8-11 which takes it out of the comfort zone of growing plants and fish together (pH 6.8) and puts it into a system not recommended for aquaponics.
So should we dismiss it as a viable system in aquaponics? Again, it depends where you live and how you manage your waste. Certainly for anyone interested in Permaculture methods that look at the broader picture, you need to look at your entire inputs and outputs and find other ways to integrate this method into your system. Here's a handy pdf guide on Growing Spirulina in greater detail.
Turning a Problem into a Solution 把問題變成解決方案
In Ferende, Togo West Africa Dr Ripley Fox turned a problem into a solution. He had the right climate and pH. He found a way to turn malnutrition in the village and marginal farming into a thriving opportunity – growing Spirulina to feed their fish grown in concrete raceways. All human waste went through a bio-digestor that also supplied methane gas and output compost for their dirt gardens. Lack of sanitation had caused disease. Now it was under control.
The villagers were able to now earn a living selling fish in the community. A problem was turned into a solution.