At first there was an iron or magnesium deficiency. The furthest grow bed is yellow and close to death.
Then I got that under control and during the past week, the growth rate has accelerated .
That marigold has been enveloped by the tomatoes.
In the pictures below there are two different kinds of duckweed growing together in this tank.
The duckweed looked like first picture taken 07/09/2012 for at least a week with very little growth. It had not changed since I started it..
In the picture the light is on, but up until 07/09/2012 I had not been using the CFL lights.
You can see the ripples from an air stone in the upper left corner. This air stone was under the raft on the left.
On the 9th I moved the air stone into the center, and began supplementing the light with several 6500K CFL bulbs.
Six days later there has been improvement in the growth rate.
On the 14th I bumped the Ammonia up a little since the Nitrate level has been near zero. Today it tested 5ppm
I think this will help to accelerate the growth rate. All along the water temperature has been about 74F.
My conclusion is that the light or the air stone has caused the duckweed to grow.