We have all experienced SAD (Sudden Avic Death). No one really knows the reasons and there are pages and pages of discussion on the subject on the forum.
I wouldn't eliminate bacteria in water as a cause (or one of many causes) but I personally don't think it's a key factor.
For you're own peace of mind you could boil the tap water to kill off any bacteria that may be in the water. Obviously don't serve it boiling tho
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