Pass,
Eric has great information there and is dead on in what he says. There are some more ideas that I have found work out and help out a lot in setting up water for water changes. First the rubbermaid garbage can is a GREAT way to mix top off and water change water! I fill mine with RO/DI water, then have a pump head in the can to circulate the water, as well as an air stone to add agitation. AFter the water has sat agitating for 24hours I add the salt for the amount of water in the can (again checking this with a hydrometer), then I let it sit for another 24hours. After this period you should check to make sure the temp of the water is the same as the tank temp, and add some sort of Ph Buffer to get the PH up to the same as the tank (8.3 in my case). You can let the water sit agitating for 2-3 more days before the water change (there is no limit to how long it can sit with the pump and airhose in it)
This is just for water changes and top offs, for the first amount of water when yo uare first setting up the tank, just put the RO/DI water in the tank, then add salt and buffer to get the salinity and the Ph correct and once the temp, ph and salinity are correct you can add your live sand. Start running your pumps and filters (and hopefully a skimmer) at this point, let it go for a week and test the water every other day or 3rd day. Wait for your initial ammonia spike and nitrite spike to fall off, then do your first water change to get the nitrates down that have developed. Then you are ready for your first set of liverock!!
Chad