Angelfish, tetras, Cories, hatchetfish, Discus. If you did lots of research and were very skilled, even a group of Piranhas would be suitable for a Amazon theme aquarium. The fish may be illegal in some areas, but if they aren't, it is possible to grow them to a medium size of about 6 inches or so and even keep similiar sized fish with them. The larger piranhas get, the more docile they become.
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If I were going to do a Amazon theme I would try to mimic a flooded Amazon forest. This setup requires a fine substrate with sprinklings of black gravel of various sizes, scattered along the bottom includes wood like Cork bark and driftwood, floating live plants and thick bunches of plants assorted around the tank create the 'flooded forest' look. Oh yes, another fish I would consider for the Amazon theme is the Arowana--these fish are common naturally in flooded forests but they can only be kept with other large fish and their eventual size must be kept in mind of almost 39 inches in some cases.
Large majestic groups of Angelfish, two dozen hatchet fish, neons, cardinals, red nose, black widows, and other amazonian schooling fish make a wonderful display. Catfish such as the very interesting Farlowella catfish would be a great addition to this sort of setup--. But of course, the good old Corydoras catfish--also from the amazon--- are still the best winners and probably the cutest of the catfish. Several species each with their own little school would make a interesting bottom feeder display. Don't forget about Discus, usually Discus don't do very well with Angelfish so they are best kept as the show fish of the tank only. I would estimate you could keep at least 4-6 along with the other fish I mentioned. Dwarf ram cichlids make a beautiful display with either Angels or Discus. There are so many beautiful exotic pleco species that can make good tankmates for oscars and Arowanas. They are well worth looking into as well.. With so many varieties--it's hard to list all the plecostomus species!
Another option you could consider---if you are really commited and want something different, is to keep freshwater Stingrays. There are a few species of Amazonian freshwater stingrays such as (Potamotrygon Spp.) all are well worth observing but require quite a bit different care. just a thought!
