One comment that might assist you. Depending on where you live, the municipal water authority might be using chloramine as a disinfectant rather than just chlorine. Depending on their chemistry, not all hobbyist ammonia test kits will register the ammonia component of the chloramine - in fact, the tap water might read zero for total ammonia when it's actually around 2.0 ppm if the chloramine was broken down into chlorine and ammonia.
While it's not optimal to use saltwater made from tap water, if your tank has a functioning biological filter and you don't change more than 10% at a time, the amount of ammonia that you might be introducing is going to be pretty trivial, and the bacteria in the tank will process it to nitrite and then to nitrate very rapidly.
That doesn't alleviate the concern over essentially fertilizing the tank (ammonia and its breakdown products are good fertilizers for algae), but it's also not likely to kill anything.
However, I would suggest that you not use tap water for evaporation make-up. Using anything other than purified water for the purpose will potentially concentrate the metals in the water, including copper.
I understand that and do not plan on using tap water for the long haul. I just have to remodel my basement before I can step up a rodi.
But as you where saying about chloramine, Marine Conditioner should be taking it out of the water along with copper. The fact that my water worked fine for months rules that out to me anyway. Only way it could have changed is if something changed with the city but I figure I should get some kind of notice before they do that.