I would recommend going with ro water from an LFS if necessary. Most towns are putting chlorine in their water now, but I don't think that would kill them that fast. The ro will eliminate you adding so many things before the water is safe. You are definitely bound to end up overdosing something or combining something that is not good for the fish. Go with the ro, test everything prior, and I would think you would be ok. I can't see normal ph differences or even alk differences of 2-3 points killing a fish like a chromis that fast. IMO it would have to be something in the water itself, not a parameter that is at least within the bounds of reality.