New here... Thinking of starting a South American-themed tank

I just moved into a small apartment and had to sell my three other tanks. I had a saltwater tank, a black piranha tank, and an African cichlid tank that just would not fit in my new apartment considering the configurations they were in.

Now I want to go out and buy an 80 gallon, but don't want the hassle of a saltwater at the moment, and would like to try something new.

I was thinking of either an Amazon-themed tank with tetras and the like in soft water, or a rainbow tank, which I know little about at this point. Any recommendations?

I don't like mixing species from different regions together.

Oh yeah, and if anybody can give me some links to information about either tank idea, that would be great.

Thanks!