an actuall Orinoco biotope would be difficult im currently making my 75 a "soulth American biotope" much easier with the rules try this website www.aquatropique.com it's in french but they biotoped their plants so you can get an idea of what speces are soulth american good luck ! AquaTropique.com here is a direct link to the soulth american section.
a "true" biotope ? or a nice planted tank with south american plants?
If you had a true amazon tank it probably would have a sandy bottom with some rocks (maybe some driftwood) and only one or maybe two kinds of submersed plants...
I was just in Brazil ....I got to see the Amazon,negro and urabu rivers and also see some wild fish (eat some too...lol)
honestly, I would go with d.gillys advice .....but then again im partial to the "