So I use small town city water not sure how to get those readings u mentioned but my fish died 2 days after the wc and my home was built in the early 20s so yes it's old I did a fairly large wc because I was rearranging rock work
Okay, so if you do a search for "TOWN NAME water quality report" on google or yahoo, you should find a pdf file somewhere online - hopefully fairly recently. For my town, Minot ND, the most recent WC report is 2018, I believe. The water quality reports are generated in labs that use good wet chemistry titration, ICP and other well-regarded methods to produce high-accuracy water reports that include things like NO3 and PO4 among many other parameters. See if you can find that. What you're looking for specifically, if you can, is what your main reef params are - like Ca, Mg, ALK (although this is usually listed in other ways like total hardness or carbonate hardness), K, PO4, NO2 and NO3.
What volume was your water change?
What I am thinking is that your salt will probably mix to values near natural seawater - *if* mixed up using water with zero concentrations of these ions and elements. But, if your values in tap are higher, the values in the resulting mixed salt water will be that amount higher as well.
For example, in my town, the tested value for nitrate (NO3) was 0.06 - so even if I mixed with tap, I would have fairly low nitrates in my new seawater (NSW). Not particularly problematic, as I would end up with NSW that has 0.06ppm of NO3 - no way will that hurt anything.
However - Ca is a different story. I use Instant Ocean, and I preach to anyone who will listen not to use anything else. It mixes up to values near what the ocean has when mixed with RODI water (I get around 9.5-10 ALK, 420 or so Ca and 1320 or so Mg. But... if I were to mix my NSW with tap water in my town, I would end up with 59+420 or 479ppm - because my town has 59ppm Ca out of the tap. Potentially more problematic is ALK. My municipality has pretty hard water - the hardness is listed at 140ppm. Total hardness includes all species of carbonates dissolved in the solution, whereas ALK refers to only the sum of the hydroxide, carbonate, and bicarbonate ions in the solution. So they are measuring different things, but can be related. I don't know of a way of converting one to the other, but I am pretty sure that were I to mix up my IO with tap water, my ALK would probably be quite a bit higher than normal - so if I did a W/C using that tap, I *could* definitely upset some of my critters.
How hgh are the chloramines in your tap? Can you taste or smell chlorine? Ever test for copper or lead? (these will be listed in your water quality report, but the values may vary because of pipes, etc.).
Have you ever done tests on your tap water before mixing up a batch of NSW? I'd test NO3, Ca, and ALK and see where your tap is at. This could help eliminate your tap water as a potential source of the problem.