Sorry but I think that every single poster in this thread thus far has been way off. In all probability, if you followed the appropriate dosage, your fish did not die from the copper treatment.
Personally, I think that your ammonia spike was caused by the piece of lettuce that you had rotting in the tank for 3 days. Assuming that this is consistent with your feeding habits and would routinely leave lettuce in the tank for several days, this is most likely what caused the ammonia.
Sorry for your loss.