Check the analysis of the bottled water. It'd be silly to exchange hard bottled water for soft tap water. If you're worried about chlorine etc, leave the tap water to stand for a day before using it.

Remember the fly needs to trigger the hairs in the trap in order for the trap to close. I can't see dumping a dead fly on a trap being very good for the plant.

It may just be that you were unlucky with your choice of plant - new plants are as likely to die because of conditions before you bought them as they are to die because of what you've done since.