Regarding nitrate problems... In the future, add new live rock very slowly, so as not to overload your existing bacterial colony's capacity for eating ammonia. I agree that purple up didn't cause the spike.
RODI units can either be for your entire house (lots of $$$) and hook into your main line, or on a smaller scale just for your aquarium water, which are what Biff & BL are talking about.
get phos-sorb or some similar phosphate removing product from your LFS and let it sit in your sump, if you have one. It will help remove some of the nasty stuff from the tap water, because even if you switch to RODI right now, it will take quite some time before all the tap water gunk is gone. Good luck