>Floating plant. Leaves about fingernail sized, slight reddish
>colouring, and often overlapping. Has flotation bladders about the
>size of grains of rice. Quite thick root system. Apparently a Tropica
>plant. Also apparently a type of ludwigia.
This sounds like a native, Ludwigia peploides, aka water primrose.
This one has flotation bladders on the root. It has very nice
yellow flowers, and I have seen huge mats of these plants covering
shallow water areas here in Texas. They bloom during the summer,
and I have always seen them growing in full sun. They're also
called verdolaga de agua.