Duckweed is an excellent source of protein that can used to supplement or even replace pelletized food for omnivorous or herbivorous fish in your aquaponics system. Indeed, duckweed is an excellent stock feed in general with chickens, ducks and many other backyard livestock happy to dine on this versatile plant with enthusiasm.

Duckweed is the common name for the Lemnaceae family of aquatic plants. These plants are considered to be the smallest flowering plants in the world and they are very simple, lacking stems or leaves but consisting of a small thalloid that floats on or just under the surface. Some varieties have a single root but not all and reproduction is mostly by budding but an occasional flower is produced that results in a utricle, an air sac containing a seed that is designed to float. Duckweed grows at an explosive rate under the right conditions where it can double its mass in less than 48 hours, forming a thick cover of 600-1200g per square metre.

Duckweed as a High Protein Feed

Duckweed is easy and low maintainence to grow and when managed correctly makes an excellent supplemental feed for plant eating fish such as Tilapea, Silver Perch and many other species. Healthy duckweed can have protein levels as high as 45% and contains high concentrations of the essential amino acids lysine and methionine. Duckweed is also high in trace minerals and pigments that are essential for the growth of healthy fish. Growth rates of fish whose diet is supplemented with duckweed are virtually identical to those which are fed only pelletized rations.

The protein levels of pelletized fish food is higher than that of duckweed and fish fed only duckweed will not grow quite as fast but still be quite healthy and productive. If your goal is to run your aquaponics system as organically as possible and at a minimal cost then duckweed may well be the answer as it a naturally occuring food that is easily grown in tropical and temperate regions all over the world.

Growing Duckweed

Growing duckweed is really easy and there are several methods you might consider using. It is interesting to note that duckweed is used extensively in bioremediation due to its rapid growth rate and its ability to remove excess nutrients, particularly nitrogen and phosphates. So duckweed can be used to remove excess nutrients from your aquaponics system, though these nutrients would be more effectively utilized in your growbed. Duckweed could be more benefical in this role to aquaculture where water could be cycled through a duckweed tank and returned to the fish tank rather than discarded. Nonetheless you may consider pumping or bucketing water from your tank into a reservoir that contains duckweed as part of your aquaponics system.

At the simplest level all that is required to grow duckweed is a pond or reservoir. Duckweed lives on the surface of the water so the depth is not necessarily all that important. I keep my duckweed tanks at a level of about 6 inches. The volume of the reservoir when filled to the desired depth is important as it will provide a buffering effect against extreme changes in temperature. Duckweed grows best between around 18 and 30 degrees celsius. Outside of this range growth may slow quite noticably and it may die, particularly below 10 degrees celsius. You can grow duckweed in buckets or large plastic tubs without any problems.

My earliest efforts with duckweed involved 100 litre tubs that were about half a square metre in surface area. As water evaporated from the tubs it was replaced with water bucketted from the aquaponics system and this system worked quite well. I found that if I used fertilizer it had to be at about 1/4 strength and the duckweed prefered natural fertilizers such as chicken manure teas rather than garden chemical fertilizer. Further experimentation showed that hydroponic nutrients in either a vegetative or flowering formula at about 6-10ppm were perfect for duckweed and resulted in a rich dark green coverage that had to be harvested daily because of its amazing rate of growth.

My current duckweed system consists of a number of tubs underneath a four foot flourescent light fitting installed with one grow tube and one cool white tube. When fertilized with hydroponic nutrients or water from the aquaponics system this setup is very productive and produces kilos of clean, very healthy duckweed over a very short period.

Any excess duckweed you produce can be dried or drained and then frozen depending on your personal preference. If dried duckweed can be converted into pellets but I find this unnecessary, preferring to drain it and freeze it in plastic bags in blocks about 1/4 inch thick that are easy to break bits off.