I find fish death to often be a combo of small things just like most accidents we hear about. But without actual test results and once things are somewhat level we can only guess. I find chlorine to be a pretty slow process, more like dying from a sunburn only on their gills? The chlorine in tap water is much the same as chloramine except for two major things. Chlorine does gas off pretty quickly so we don't have to treat water with it in the tap. We can actually just wait and it gasses off. This is the way it used to be done. But chloramine is designed to stay in the water with the advantage being that much less of it is needed to maintain safe levels all the way to the end of the lines. If using chlorine, we may need to add the max allowed 10ppm at the plant to get the minimum allowed 3PPM at the far end but using chloramine we might add 5 PPM at the start and still get the needed/required 3 PPM at the end. It is safer as we use less and it lasts longer but that does complicate things for us so we use Prime, etc. But even if we overdose, it is almost always safe so there are probably other points involved when fish die quickly.
Ammonia kills much quicker than chlorine so that is where I often look. A sudden rush of ammonia can come about if we do some simple things like pouring the water in quick and stirring a bunch of the substrate. This can do two things, one is it can kill a portion of our bacteria that was living on the surface of the sub but it can also stir a bunch of gunk that was below into the water column where it becomes ammonia. Pouring in from buckets, this might be easy to do?
Was it also time to clean the filter and wipe down the walls or maybe move some plants? Doing any one of these will cost you some bacteria so if we do several things at one time , each will put us a bit closer to an ammonia spike. That spike can be a rolling factor that comes through strong enough to kill fish which may be weakened from pass exposure but be gone when we get around to testing.
Water temperature changes can also be a hazard as harmless ammonium can become hazardous ammonia at different temps.
So I would review what combo of things might have happened as the single point of overdosing is pretty uncommon for quickly killing fish.
I find one of more common causes of fish death is trying to do too much, too quickly and that combo can push fish that look fine over the edge to death.
But testing is about the only real way to get the answers.
For the water report, just find what company provides the water then it depends on what size how that info is passed small can just post it on a bulleting board while larger are required to do more to make it available. Many times a search for Consumer Confidence Report (CCR) and the name of the supplier will find it online. But then do keep in mind that it will only give high, low and average on many points so it does not tell you what your water has on any specific date.