Although they don't die directly from Phosphates themselves, it was the outbreak of cyano that probably did it as it can take the oxygen out of the water, therefore leaving poor water quality. I think at the time I also had a problem with a temp spike where it reached 86. I lost quite a few fish.
But we digress slightly.
Sam if you can tell us your acclimatising procedure and your params this might help us with yur problem. Also what size tank and what other inhabitants. Anything to give us a little background.