Bummer Jose...
Jose theres got to be something wrong with your water...their behavior in the movies was fine... to die unexpectedly now makes no sense.
It may help if you go into exactly how you prepare your water for changes.... I'm also concerned about your biofilter... Before the fish arrived there was a biofilter issue..I can't help but wonder if this is related.. I know you posted zero ammonia and nitrites...but if the biofilter was impaired ,,they may be there...the kit may be expired.
-al