Personally, I'm not a fan of the purple up. My hubby wanted me to use it a while back because he wanted rocks to be purple faster, but I only did a couple doses and then stopped. I don't like adding anything to my tank that isn't necessary. So I don't know if it works or not.
As far as the PHs, in my my 55 I have two 700gph and one 500gph, so you adding an 800gph definitely won't be too much flow.