When I do a water change, here is my process.
I have 2 trash cans. A good water bucket, and a bad water bucket. The two are never mixed. I have "good" on one, and "bad" on the other. Since I do 40 gallon water changes, I use a 45 gallon trash can.
Fill up a trash can with water. Pour the salt in. Guess-timate it the first time. Put a heater in, and heat it to the exact temperature of the tank. For the first 24 hours, put a powerhead on the bottom of the bucket/trash can to mix the water with the salt. Salt will fall to the bottom, so this is where you want your powerhead. After 24 hours has passed, bring the powerhead closer to the top to increase the oxygen. Dissolving salt with water uses the oxygen up in the water, so you need to replace this. You could use an airstone, but I just use a venturi powerhead because it sucks air in (I run 2 poweheads, one at the top, and one at the bottom, since I have such a huge trash can - you only need one). After the first 24 hours has passed, check the salinity and pH and adjust accordingly. If it is too high, add regular water. If it is too slow, add more salt. If your pH is different from your tank, check your alkalinity. Many times, you need to buffer alkalinity because there are some salts that you give a low alk level.
Once the salinity, pH, kH, and temp are EXACT to your tank, you are ready to do a water change.
When I do a water change, I never dump water into the tank. I have a 5 gallon bucket that I use to take water from my trash can. I put the five gallon bucket on the floor and pump water from the 5 gallon bucket to my sump using a small powerhead and some tubing. This way, the fish are not getting stressed out by you adding a lot of water to the top of the tank. They do not even realize that you are adding water to the tank because it is calm, plus all the water levels are identical to that of the tank. (Plus, this way is a lot less messy).
If you have any questions, please feel free to ask me.
