Saw this Praying mantis sitting on a log When this wasp started annoying it, Then Bam! the Mantis Just grabbed it out of mid air and started eating it Alive.
Managed to get this Pic, Ive got some more that i might post later.
pH shock would be my guess. You didn't aclimitise them long enough.
-I would put the bag in there cube tank and slow aclimitise them over the period of an hour. Every 5-10 mind i would add a cup or two of the tank water to the bag.
-You also might want to do a pH test on both the bag water and the tank water and see what the difference is.