LFS owners will tell you something simply and it keeps you buying as far as their concern, for as so, their next thought to what is wrong will be that your doing something wrong. Do understand this that your LFS lively hood relies on you buying their live stock. I not know to many LFS owners who will tell you what is be in you making the right selection to beginning your tank to age, for this is what is yet to happen.
Let us recap some details here, you cycled your tank without fish, so your still not with a well age tank.
Adding fish without respect to the tanks' age: Adding fish during the break-in cycle, not using hardy fish for the break-in cycle, and adding fish that need a well aged tank too early, will almost always lead to disappointment.
Now im sure your LFS owner understand that a tank that isn't well age as yet, will or could lead to certain deaths, lost of fish. If you need to ask certain questions about your tanks conditions or additives requirements, your really best to ask here and not your LFS who gains from your lost for we gain much more in your possible success in this matter.
So once again I say your lost was due to a not so well age tank, its my .02 worth of data to what you need to follow up on. And as the girl I told you about, she too waited until her tank cycle completed then bought the few chromis against my advise not too, but yet some, few do have some success with chromis during the early stages of their tanks, but not everyone.
The bottom line to this whyzee250 is, I don't buy it that its always the fish, still, it could be as so, for your LFS salinity is around 1.019 if not lower and if your tanks salinity is much higher like around 1.024 or so, this could cause extreme stress on your fish.
Its like when the time comes for me to start my SPS tank and a QT as well, the main tank will be at 1.025-6 and the QT will be at 1.022, not to cause any more stress then can be help with.
Buddy
Happy New Year