After only one day as usual delivery time my Item arrived - and lo and behold, when "Extra" there was a free sample of "Tetra Safe Boot" (30ml).
Following instructions (5ml of plant fertilizer per 10 liters of aquarium water) of the plant was the same with the fertilization started and lo and behold, the plants grew like the devil. My waterweed I had to prune twice a week, so they do not fully proliferated throughout the basin :)
As my plants (after six months) mitterweile still grow very well, I fertilizing only every second (recommended by Tetra once a month) to third water change after.
I can only recommend this plant fertilizer. The plants will thank with lush growth and bright green !!