If you have soft acidic water PLEASE South American Biotope. Those tanks are gorgeous and so interesting! And in a 90G??? Man, you could have an amazing setup... You could have angels, with fuller-bodied tetras, or smaller tetras with SA dwarf cichlids, Corydoras, Otocinclus. It's one of my favourite tank setups.
I was thinking of something similar to a South American biotope. But I have limited choices because of my discus and I don't want any other cichlid in the tank besides him. He has to be the star. I forgot about otocinclus, thank you for reminding me, definitely have to add some.