When you say big water change, how big of a water change in a 15g ? SPS are very sensitive to swings in parameters, changes in GFO, Lighting, flow. All of these and more need to be stable, so when adding, changing, or shifting, be sure to do it slowly. I've found birdsnests to be sensitive to swings in alkalinity, so test daily for a couple weeks until you figure out the demand of your tank, and get the 'feel' of your tank. As you add more livestock, and sps especially, it will shift your balance in your params, so check, check check.
Try to acclimate the coral to your lighting also by lowering the coral, or lowering the hours of your lighting period