when I started my 75 gallon I just mixed RO water 5 gallons at a time in a home depot bucket and added about a full Ikea cup (lol) each 5 gallons for the first few dumps into the tank. with each bucket I heated up the RO water to about 100 degrees to help disolve the salt. When the water was about 1/3 full in the tank I started to test the Specific Gravity and Salinity each 5 gallons I did and adjusted how much salt I mixed in. I left about 10 gallons not filled up at the top so when i dumped my live sand it it would leave enough room, also so I could adjust my SG and salinity after the water settled and cleared up... it took about 22 hours for the temp to get down to 75.5 degrees which for the most part that temp stays really constant in my tank without a heater. I have a heater in there but it has yet to turn on, at first i thought it was broken but I put it in a fresh water 2 gallon jug of water that was in the fridge and it heated it up to 74 degrees which is where I had it set to. So I guess I am pretty lucky that the heater has yet had to turn on in my tank yet.