Good stuff, Sam!
This is why preserving / conserving habitats is so important...vs. fish by fish.
A group of fish that are fascinating, unique and beautiful (in their own way) are the cichlids from Lake Barombi-Mbo Cameroon.
Here's a good article on them from cichlid-forum:
http://www.cichlid-forum.com/articles/Barombi-Mbo.php
I've kept a lot of these guys over the years and have really enjoyed learning about them and the little crater lake from where they originate...
Matt
PS On a slightly more cynical note, some of the rarest fish in the hobby are those that are maintained well for a good period of time. Just about any cichlid can be a masterpiece if given the right conditions and care from its owners!
There are a whole lot more freshwater species that are endangered that don't appear on the list. You could check the IUCN Red Book but I'm guessing that it's way behind the times for the Amazon - there's a lot of stuff down there that gets erased before it even gets catalogued.