Your water change is the same as in fresh water. Remove x amount of water replace it with the same amoumt, so you keep your salinity the same. I have a bunch of small starfish and one from the last group buy. Chocolate ship starfish are not reef safe. You should read carefully because some require a mature sandbed and some are pretty predatory. You are doing the right thing by reading. Also all starfish require a good drip period no shortcuts.