People ask all the time. “How do you grow duckweed? I tried and nothing”.
There is no secret. Duckweed needs nutrient rich water to grow. With the right conditions it will double in volume every 48 to 72 hours. Once it gets started it’s hard to stop. The concept of aquaponic is plants thrive on fish waste. In the case of duckweed the thicker, the nastier, the better. However I have heard that some people are having “success” by adding sports drinks. Not sure if that works, others have used a plant fertilizer, but don’t put it in your fish tank. It needs to be in a separate tank.
I moved my duck weed farm to another location and let the tank dry out and as you can see, the bottom was pretty nasty looking. This was one of my best producing tanks.