I change 20% every 4 or 5 days on my planted discus/angel tank.
I treat the water before I add it. I do not like treating as I add the water or treating the receiving tank prior to adding the water.
I actually have a separate tank that I use to prep water for water changes that is heated to the right temp. and treated with prime. I also add the plant food and flourish excel to the changing water right before I perform a water change. I only use the gravel vacuum ever couple of water changes.