if the water quality is not good your old fish will be used to it as the change has probably happened slowly over time. the new fish however wont be used to it.
what bumps fish off is the sudden change in things, ie temperature, water quality etc.
when you get your new fish i used to open the top of the bag and float the new fish in their bag in the top of your tank for a couple of hours so the temp in the bag equalises with the temp in the tank slowly. also put a little bit of your fish tank water in the bag, then an hour later a little bit more and so on. although this is still a fairly quick change in the fishes eyes a slower introduction to the new environment might help them cope better.
best advice - before you buy anymore fish get some water quality testing kits and test your water. you should be doing this at least once a month anyway. i know its exciting setting up a new tank and you want to rush out and buy things but taking your time to set it up slowly and properly will pay off in the end.