Have a look into the three habitats that the Amazon river system offers, blackwater/clearwater/whitewater. Each has a slightly different composition ranging from soft acidic PH 6.0 in blackwater to harder more alkaline PH 7.2 in clearwater. Blackwater - full of rotting leaves and bogwood, Clearwater - filtered out of mountains with lots of mineral content, Whitewater - muddy water, with a composition similar to whitewater.
Discus are more at home in blackwater conditions, as are blue rams. Corydoras inhabit them all, but some species prefer one to the other. That all out of the way, and if you don't mind mixing localities, most fish are comercially breed and will withstand most conditions as long as they are stable.
Jon