I always mix my new water for at least 48hrs, I know this may sound extreme to most, but I had a few instances when I was just starting out in the hobby where my salt was not completey mixed, thus causing cloudyness and salinity jumps. I also agree with Mak, I think a 40% change is too much for mixing two new salts, maybe this caused some kind of chemical reaction- like the one that some are reporting with Crystal Sea and I.O.
Megalodon: Sorry to hear of your situation
Hopefully J&L's test is positive and maybe that way you could in turn seek some kind of damage claim from I.O. for your losses. If I were you I would be sure to save all specimens and also take pictures as well. Sounds like maybe there was a mixing or settleing problem
cheers, Rich