i'd take the fish back for a start! try getting one from elsewhere and acclimatize properly as suggested ad water to bag little and often, sorry for your losses!
Mine grew from about 9" in the year I had him until last night when it had a stroke or a fit or something and it went into the side off the tank so hard that it killed itself.
Nat is probably right we have a friend who sold some discus to a guy the change in the ph almost burned the discus instantly. Also if buyind nics buy females they have the best color males are very bland.