If you have municipal water, it’s possible that you’re getting other contaminants in your water from the countless miles of pipes that the water goes through before getting to your home. Nothing from your copper pipes will leach into the water with a pH that high. Do you get a stain at the water line in your toilets? If so, what color is it?
Although I would be concerned about whatever was causing the film in your water, changing your pipes is going to be very expensive, and it might not solve your problem if it’s coming from outside of your home. It’s a lot cheaper to get an RO system. That’s what I use, as do a lot of growers. It’s relatively inexpensive to acquire, reasonably simple to set up and not a big hassle to maintain.