I have always loved amazon biotopes, particularly Blackwater Creeks. These creeks are Acidic (PH 5.5-6.5), and are generally devoid of plants and Rocks, and the substrate is usually clay based with an organic mulm coating. They are normally bordered by semi-aquatic plants which are generally submerged several times a year. Of course there are also other Amazonian biotopes (Whitewater Rivers, Clearwater Streams, Oxbow lakes, to name a few).
Here are some inspirational pictures: