It is the first one you are linking to (Lemna Minor).
Pretty common over here, but the cold winters keep them from exploding.
It is one of the fastest growing plants, and can double their biomass within days. It contains 20-40% protein, and are one of the calsium-richest plants known. It also contains vit. B and C..
There are research going on to try to use it as protein source in salmon-food.
It is absolutely brilliant as food for egg laying chickens because of the high calsium content (10-15g/kg) and it also contains xanthofiller and caroten, wich gives nice color to the yolk.
One of natures little secrets