Welcome to the Malta Aquarist Society Forum, joseph. 
I think that when buying fish, you need to place them in a quarantine tank for a few weeks. For sure i cannot say what happened to both your Discus, but if the new one had something, it might have passed on the other one that you already had & so both died. With a quarantine tank, the story would have been different & only the new one would have died.
How long have you had the first 4 Discus?.
Please treat Corydoras as community inhabitants & not as scavengers.

