Here is what I do, step-by-step exactly:
Equipment needed: extra heater, 1-2 small powerheads, a "good" and "bad" trash can that should only be used on your tank, and your synthetic sea salt.
1. Two days before I plan to do a water change, I take my "good" trash can and fill it with the desired amount of water. In my case, it is 40 gallons, since I do a 20% water change on my 210. Thus, I fill my 45 gallon trash can up, pour a ton of salt in, enough to make it have a salinity in the range that I am intending, and then I throw the powerheads and heater into the trash can. I set the heater to the EXACT temperature that is in my tank. I do not touch the trash can for an entire day. I let the powerheads mix the water up and dissolve the salt.
2. The next day, I go to the "good" trash can with my refractometer and check what the Specific Gravity is. If it is too high, I take water out and replace with freshwater. If it is too low, I add more salt. I continue to check on it throughout the day to try to get the SG to the EXACT SG as there is in my tank. Before I go to sleep on day 2, I make sure that the salinity is exactly where it should be so that I do not have to add salt to the trash can the day of the water change. I also check the pH and alkalinity of my water to make sure it is where it should be as well. Sometimes, salts have a low alkalinity, so you do not want to go without checking this. If either of these levels are low, correct them before the day is over so that you have a full 24 hours to let any buffers dissolve before adding the water to your aquarium.
3. When I wake up the next day, I know that the temperature and SG should be exactly where they need to be because I fixed both of these levels the day before. I then take my bad trash can, take a siphon, and go over to the tank. I take the good trash can over as well. As I siphon out water and vacuum my aragonite (since I have a FOWLR tank), I have a pump in my good trash can pumping my good new water into my sump. Thus, my water level in my sump changes very little during this entire process. As I am doing the water change, I take another powerhead and just blow the powerhead over the surface of my live rock in order to get any settled debris off my rock and into the water column so that it can be picked up by my filter or siphoned out.
4. After I have removed my 40 gallons, I change my filter bags because anything that I did not siphon out was picked up by my filter.
After you are done your water change, all of your tank levels should be the same because you added water that was of the same temp. and same SG as that which was in your tank. Thus, your animals will not experience any negative effects or long term stress due to this water change.