It is Hydrocotyle Leucocephala which I use as a floating plant to provide hiding places for fry and smaller fish. It also provides shade for the less light loving fish.
I am not sure if it is a type of lily - the leaves look very similar but I am no expert. Apparently it is used as a herb in the tropics.
It grows really fast once established - that it about three or four weeks growth and there were several more hanging down the back of the tank as well. It seems to last OK outside the tank but looks a lot better (and healthier) on or in the water.
It can also be planted in the substrate when it will grow up towards the light and then spread out on the surface - I might try this when I revamp a small tank in the near future (snail problem).
Cheers, Eddie