IMO, RO/DI is important in this hobby. You will be replacing trace elements when you mix your saltwater (assuming you use artificial salt mix). As the trace elements are used up, they get replenished with water changes. There are far too many nasties in tap water, IMO. I use RO/DI for all of my top offs and salt mixing. With your tank only being 6 weeks old, it will go through algae stages, just keep on top of your water changes. What type of algae do you have growing?