I think 1/2 cup of water every 5 minutes is too much, too often. If you're using this method, perhaps less water every 15-20 minutes would be better. Just keep an eye on the temperature.
When you were floating the bag, was it still unopened?
And, as Steve says, find out the salinity the fish came from so that you can adapt your methods accordingly.
Then again, the fish could have been doomed from the start. It may not be the acclimatisation that killed her...how long did your LFS
have her in before you bought her?