If there is sulfur dioxide in the air and it's mixed with water and oxygen it will create sulfuric acid and tarnish your nickel plated items. Just on the inside you say, and when the shower is turned on and tested you end up with acid rain. Are there any manufacturing plants close by? Any smells like burnt matches? Not saying it is sulfur dioxide or not, I'm just throwing out a possible reason.

The shower has never been on. All the hardware thru thru the house is polished nickel - nothing else has discolored. Given its immediate discoloration, my feeling is that it had to be something in the installation that caused it.

Are the shower doors open or closed? If they are closed, there could be off gassing by the shower tiles or grout, or gasses coming up from the drain. I'm saying this because you said the side facing the room was fine; it was only the side inside the shower that had the problem.

Good answer louise! If the shower has never had the water turned out perhaps the traps are still dry. I believe chloride is used to darken nickel and PVC is polyvinyl chloride, and PVC glue is made from methyl ethyl ketone, cyclohexanone, tetrahydrofuran, and acetone. Sewage gas will have hydrogen sulfide, ammonia, methane, carbon dioxide and sulfur dioxide to name a few. Something is causing the nickel reaction.

The shower is not totally enclosed as there is a gap at the top to allow for venting. Door has been more open than closed. Also no other hardware (including shower hinges) inside the shower has discolored. And the discoloration on the clips has not gotten any worse since the day installed.