Randys 2 part has been around for a long time... that is where BRS got the recipe from I'm sure...
With that a few years ago I think there was a change with the dow flake(I think it was the dow flake) and how it was manufactured. Randy did some work to try to find out if the new ingredients/processs in it would be safe for tanks and I'm not sure what the outcome of it was.
Edit1:
Here is a post from Randy on another forum (sorry for linking to another forum but I think this is important):
"You can now buy bulk calcium chloride from BRS and Dr Foster and Smith. "
http://www.reef2reef.com/threads/dow-flake.180566/#post-2076383
Edit2:
He does say he would still use the dow flake though:
"I'd probably use it, but slowly at first to watch for unexpected effects. "
http://s.reefcentral.com/forums/showpost.php?p=22763179&postcount=9