If you watch the videos of rays in the amazon you will notice that the substrate is usually covered with a layer of detritus and organic debris, leaves, etc... silt is very soft, not sharp like the pictures posted on page 1.
African Cichlids have evolved in an unchanged body of water for billions of generations in which they adapted to sifting sharp sands with their gills..
The amazon is so diverse and changes so often and so rapidly, to say that they 'were made to live on sand' would be completely way off base.. Just because they have the ability to burrow themselves does not mean they were made to live on sand.