Some people like to prepare water in advance that the auto fillers pull water from. Others prefer to have RO
water drop straight into the tank.
Myself, I'm more of the "I'll mix is myself" and then put it in once a week. I've a sump just for that reason to keep the main tank at MAX water level. I can go at least a week maybe even pushing two before I need to add 6-12 gallons to the sump. (All pumps are at the bottom of the tank.)
75gal Salt, 18gal Sump, Skimmer, 60lb Live Rock, 90lb Live Sand, 3 Pwrhd, 2x55w Actinic, 2x55w 10k. Guest list: Goby Green Clown, Percula Clown and entire clean up crew.