In my small setup, I have used between 3-6 liters pr. plant including the water in the pipes, see link eg. which is okay, but in my experience the more the merrier mate.
Due to the fact that it is easier to keep the water temps. down with more water in order to keep algea away, next time around I will have 8 plants with a total of 60-70 liters of water, can be troublesome with water as low as 3-5 liters as mentioned.