Huh. My first question is whether it was the tap water that killed them? Fentanyl is pain medication right. You'd think people would notice something was up before they _die_...
Yeah, that's the problem with tap water in particular, it's not sterile unless treated. Also, in an IV (or an enema) you're running the risk of hyponaetremia, hence why 0.9% saline solution is the default hydration IV fluid unless you have a condition (usually kidney related) that requires a lower concentration.