Yep get rid of that mix it'll just kill any that you put in it. Instead of regular foam box's go to a pet store and see if you can get some of their fish shipping containers, they're a heavy duty foam that's sealed and when surrounded by ice packs or in a fridge work great (from Lunkers Love Nightcrawlers, Fishing Facts book).

I don't know if it was so much the heat that got you but too many worms in the box!!!!! which ate up all the nutrients and just overcrowded the box?? It only takes one dead one to wipe them out.

Fatman