There are several South American habitats you could try;
Whitewater River:
These waters are muddy due to the high amounts of sediment, this means the light levels are low so there is little to none plant life. These waters normally contain larger fish species like Piranhas, Oscars, Geophagus, Pimelodids, Doradids although smaller species like Tetra are found in such enviroments.
Clearwater Stream:
These waters usually drain from the highlands, the Rio Xingu is a good example, the water is clear and resonable transparent. Pike Cichlids, Loricarids, Corydoras aswell as some species of Peacock Bass are found here.
Blackwater Creeks/Streams/Rivers:
Acids are leeched from decaying vegetation creating very transparent, tea-colored water. The substrate is normally decaying leaf litter, meaning the light levels are reduced and plant matter finds it harder to survive, these waters are normally slow moving. This is where alot of common community fish are found like Angels, Tetras, Hatchets, Corydoras and Loricarids. However, Pimelodids and Discus aswell as others can be found there such as Apistogrammas.
A nice set-up might be a Blackwater set-up containing Banjo Catfish (Dysichthys coracoideus), Apistogrammas, Helogenes marmoratus and a few others. Or maybe for something abit more interesting a more Oddball themed tank containing South American Leaf Fish (Monocirrhus polyacanthus), Banjo Catfish (Dysichthys coracoideus) and Twig Catfish (Farrowella species).
If you moved away from the biotope theme you could also try some other small odd fish like freshwater gobies, african butterfly fish, pike livebearers, frogmouth catfish etc. Good luck.