If you are getting them all from the same source, it may be that your water parameters and the LFS parameters are too far off such as salinity and pH. When asked about quarantine, you stated 30 minutes, I am assuming that is the amount of time you float the bag? If so your clowns can not handle a large pH or salinity shock all at once. You might want to get a sample of the LFS water and compare all of it's parameters to your own. For a new fish to just go to a corner and die quickly sounds like a pH shock problem. And also use the drip method to acclimate.