What went wrong in my fish tank?

I relocated my 135 gallon fish tank this morning. I started by draining over 50% of the water into tubs to be reused. I put all my ciclids (7 total) into a tub until I moved the tank. I filled the tank with 50%old water and 50% warm tap water and after it ran and mixed for a hour or so I put my ciclids back into it. I just got home after being out for the evening and everything is dead except my 3 oscars (cockroaches of the fish world I guess) all my African ciclids croaked. what did I do wrong? was this shock or the water change?