You said you adjusted alk with a water change. Do small water changes (but more often) a large water change can cause an alk swing among other swings (temperature, salinity, etc)shocking sps coral. Any alk swing is bad we know that for sure and in my opinion alk should slowly be adjusted only with a drip doser .2dkh max per day in an sps tank. Any fluctuation and i see problems though. If it doesn't test the same every time I get worried. Also leaving skimmer off can make the pH fluctuate as the tank isn't blowing off CO2? I don't know a lot about that but it might be something you want to look into. It's so important that some people run skimmer intake to fresh air. SPS tanks that have the harder acros require a lot of attention to detail to keep colors and keep stuff alive it's a fun challenge