A salinity jump like that, yes. Slowly, WITH a refractometer, and by topping-off with fresh ro/di water (or just fresh dechlorinated water if you don't have ro/di), to make up evaporation, bring your salinity down to 1.024 for your surviving fish no more than .02 change per half hour. Then go to the New to the Hobby section of Reef Central and read all the posts that sit permanently at the top of that forum: those will help you a lot. You need a refractometer, (cheap, from Amazon) and you need some info from the posts I have pointed you to. Your fish arrived probably at a salinity of 1.024 and your water sits at 1.030, and that change happening fast is very likely the cause of death: kidney failure. If you don't have an auto topoff system, which keeps your water at a stable salinity, that is a problem. Bouncing salinity up and down is a problem for a tank.