A biotope sounds great... but for one consideration. Not all Amazonian fish are coming directly from the Amazon anymore, so the safest thing to do when adjusting/fixing water params, is to find out where the fish are coming from and what conditions they were bred/kept in before you bring them home.
Because we've already touched on the topic of angelfish, lets stick with that for an example. A wild angelfish (otherwise listed in the LFS's as "altum angels" generally are F0 - F1 generations, so are likely coming in as wild caught or are the first generation of fry from wild caught. Their bodies will be mostly adjusted to the Amazon conditions... very soft water. Now, standard angelfish (silver, white, solid black, marble, etc) are primarily coming from breeders, are way beyond 5th or 6th generation from the wilds. These fish are quite often bred and raised in water than is much harder than the Amazon environment. Not all fish can tolerate being acclimated to different conditions, and the bigger the difference, the harder it would be for them to try to adjust.
When setting up your biotope, a few things I'd like to ask you to think about long and hard before doing anything with your tank...
1. Can I create an Amazon "biotope" without harming the fish that are captive bred?
2. Can I set up my biotope to work with Amazonian animals, plants, etc.. without matching the water params of the Amazon River?
3. Is it possible for me to find out if the fish I want are wild caught or captive bred?
I have noticed there seems to be a lot of interest lately in creating biotope situations in home aquaria. There is nothing really wrong with this idea provided you understand the different considerations before getting started. I have seen many people limit themselves to very few animals in a very large tank simply because their conditions couldn't provide for captive bred animals, which in freshwater, is the majority of what is available in the LFS's now. It is entirely possible to set up an aquarium to simulate an Amazonian habitat without messing around with water chemistry. There are a lot of common plants from the Amazon that can also no longer handle the wild conditions... for the same reason. Evolution has everything to do with it, technology alongside of it. This is what our scientists and govt's consider to be progress... even if some of us don't agree.
Sorry for the lengthy posts, I just felt it was important for you to understand what you are dealing with here. There is another member working on creating a saltwater marsh, natural habitat... but is finding that the silt mixed in with the sand is also part of the habitat, and isn't something that can really be filtered out of the tank... and if/when it is, he no longer has a completely natural biotope, or a situation where those wild caught animals can survive long term.
The Amazon has a number of different biotopes. Before deciding what you wish to do, it's a good idea to get a feel for each different biotope. The Amazon River, for large part, is very muddy/silty. This natural habitat is going to wreak havoc on filtration units... but is needed if you really want the entire biotope situation.
I hope I am not discouraging you... I simply want to help see this go right for you. There are so many things to worry about when dealing with biotope situations the way I've heard it explained here and from a few books. (No, I would not suggest those books to anyone because it would take a true scientist to pull it off). There are
ways to do this... its just very important to understand what works and what doesn't, and why.
If you need more help, suggestions, please... ask away.