The fish that are the problem there are the Knife fish and the Leopard Plec both large growing fish, it dosen't look over stocked to me. Personally I would move the Knife fish or the Plec and get some more Discus. I am planning on buying one, two or three Discus for my 180L South American tank and that is alot smaller than yours, although it dosen't have any monsters just one Angel and a shoal of Tetras that inhabit the main water levels all the others are normally hidden.
Did you buy a pair of Dicsus or just two seperate ones of the same size?