How did you add the ph adjuster? Do you use rodi water for topoff and to make your salt water? How often are you doing water changes? While damsels are small they can be quite mean. Waite a few weeks before trying any more fish and do a 10-20% water change per week until then. When you're ready to try another fish, observe it in the store for 10-15 min. Look for: is it swimming well, not just moving but does it have vigor and is it strong (good) is getting along with others in the tank (good) or is it being bullied and cowering in the corner (bad), or is it being aggressive towards other fish (bad) and always ask to see the fish you want eat. If it doesn't eat don't get it. The stresses of being captured and passed between many tanks the weeks leading up to when they arrive at the lfs can be more than some can handle stress wise. Before you add another fish move the live rock around in your tank so that the damsel doesn't have the home field advantage with territory. You always want to start with fish that are smaller and less aggressive as well. Also do a quick web search either before a rat the lfs and search "problems with....(sp. you're looking at)". I once bought a 6 line wrasse cause the lfs said they were great fish for the reef. If I would have done my homework I would have learned about 8 out of 10 6 lines are mean. It took me 3 weeks to catch that mean little sucker.
Remember nothing good happens quickly in this hobby so dont try to fix your parameters quickly. This site is a wealth of knowledge and everyone is so friendly so before you decide to alter anything in your tank do a search or post a thread and learn from others mistakes. Then you can approach your tank with greater confidence and be able to provide the best possibly environment for your fish and have a much more enjoyable experience in this hobby.