>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.