Personally, when I filled my 120, I made water in 30g batches and added it as it was mixed and ready. Took a day or so but I had live sand so I added the mixed saltwater, a powerhead and a heater to the tank as the other was filling/mixing. Nothing wrong with mixing in the tank as long as you aren't adding live rock or sand before the water is properly mixed but adding sand/rock to a tank of water is more difficult IMO.