tangent: I’m in Chicago as well. LFS told me tap was fine for a fowlr tank as well. Shocked me given all the info on reefs. Looks like my planned fowlr is its own beast in that regard. Thoughts?
I'll speak from a scientist's perspective first. There is certainly evidence that certain contaminants can cause harm to corals, for sure, and they can be quite sensitive at times.
However, too much of anything can kill anything, including us. Too much oxygen can kill us. Same with food and honestly anything that we take into our body.
Now, I have not seen a study where the effect of salt water made from tap water on corals and fish have been conducted. Given the vast number of studies on corals produced constantly, if there was a study I may have just missed it though. But then again, interest in answering such a question specifically by scientists is probably pretty low.
I have not seen robust experiment done by an aquarist to investigate something like that. Again, could have just miss it.
So as far as I am aware, there is no concrete, clear idea of what in tap water harms marine organisms, and how. Except copper + inverts, which was pretty clear. But other things, like using tap water + corals, honestly I am seeing less and less evidence that there is generally (i.e. not the uncommon case where one's tap water just has so much contaminant) harm caused by using tap water. Honestly, a lot of the toxicity attributed to using tap water could be due to coincidence or secondary, indirect effects. For example - an aquarist using tap water may also be less 'careful' overall at taking care of their reef, hence it's less the tapwater and more their husbandry practices than anything.
Anyways, whatever. I'll just speak from my experience.
I don't use tap water in my reef aquariums. Not once have any issues re: corals could be attributed to the usage of tap water.
I had corals grow plenty well. Same with fish.
I am not saying tap water is always safe, just that the dangers of it may be overblown. I really do want to stress this is just from my experience, and hey, I may just be lucky. The tap water of the six or seven or so places I lived in may just happen to be safe enough, for example.