Skip discus, a 40 is too small.
But a blackwater SA biotope would do nicely.
A shoal of tetras, a pair of apistogramma, and a shoal of corydoras would work out well, but be sure to check your GH and the GH of the fish you're looking to keep. Most SA fish need GH under 215ppm, or even lower than 170ppm, and over time if kept in the wrong water can be weaker to problems and not live as long as they could.