that tank is quite tall so something along the lines of angels, gaurami or your original idea of discus would work as they will fill the "space at the top" you will have if you don't get them. then you could add the middle & bottom dwellers to complete the equation. I had 4 angels in a 1000mm tank & when they became adults it was no where near big enough as they faught so much. they were fine until it came time to breed so you can have a few until then & there isn't usually any probems selling adult angels if thats the way you go.