You changed everything all at once?
My guess is that you broke the nitrogen cycle.
(read here
http://www.fishlore.com/NitrogenCycle.htm)
Your tank started over, and those fish likely died of ammonia poisoning.
Did any of them have red spots on their gills or bodies?
It took about two weeks for my guorami to die of ammonia poisoning, so three weeks is about right.
You should really change the water in increments. 50% at a time is the most I ever do at a time, though even 75% is probably safe. 50% a week is considered an excellent amount, no matter how you break it up. (all at one time or 10% on five different days)
Also, did you change filter media or sanitize the thing when you changed the water? Even with a 100% change, it seems odd for this to have happened.