Hey, just a thought, most times you order fish you will get extra because its common to have some die in transit or in the next few hours after receiving. Of course they may not have had enough oxygen, they may have gotten ammonia poisoning but what also might have happened is PH shock. Not saying that your ph was wrong or that even theirs was wrong, there is a range that can work, but the difference between the two might have been too much for some to handle plus all the other stress. before ordering fish you should always find out what PH their fish are at, so that then can be acclimated to your ph or if you have no fish yet, you can change your ph to match.