though you bought the seawater it is still better to let it run for a while so as to cutivate the beneficial bacteria. acclimatisation of the livestock will have to be implemented .also test your water before you add the supplements. over dosage might also be lethal.
some fishes are naturally hardy, and the fishes in your tank now would be considered hardy. and a fish that has been in the tank longer would actually be more accustomed to the tank condition than newly introduced fishes. so its not that suprising that the 3 mth old fish is fairing better than the new ones.
seawater in my opinion would be better than your own salt water mixture. and a 100% water change would be too drastic. maybe you should do it gradually.