Eddie, when its time for a water change (either a scheduled water change...like one a week...month etc or because one of your parameters are off, you
make up some salt water that matches the salt level in your tank (or maybe a little more salt, or a little less if you need to adjust the salt
level in your tank a little). You can siphon or pump out the same amount of water out of the tank that matches the amount of new saltwater
you have made..say 5 or 10 gallons. Then you can replace that water with the new water you have made up. Make sure your new water is also close
to the temperature of your display tank.
Your ahead of the game by asking questions on this forum, I also suggest you do lots of reading about setting up and maintaining
a reef tank. There is lots of information online to help you out and of course lots of people here who will answer your questions. :bigsmile: