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That probably means you aqaurium is 120 x 40 x 40 cm? In my opinion a bit too small for discus fish. Th apisto's are not a problem and could be combined with two or three other species, when the tank is planted and full of hiding places. Discus I would not do in this size. Minimal 50 to 60 cm high and 120 to 150 cm in lenght.
I am geeting the tank off a friend (Kingborris) who already keeps discus in it, its not the size you mentioned, cannt remember the size now, but he has it with discus in it now and they are happy enough