A typical method of introducing new fish includes floating the bag in the new water for an hour or so to equalise the temperatures, then adding a bit of new water to their bag to introduce them to different pH etc. than adding a bit more every half hour for a few hours, then opening the bag and letting them find their way out by themselves when they are ready.
If the people that sold you the fish didn't tell you that stuff, they should have.
It's unlikely that anything toxic in your system would kill them right away unless you accidentally filled the fish tank with toxic waste instead of water.
It sucks when fish die
