DOM, I am afraid that the simple excuse of changing water parameters is something your LFS will use to deny any claim that you have received inferior stock from them. I find that any good fish, like fish I get from a club auction, can readily adapt to my water. The ones that do not adapt readily have something inherently wrong with them. They may have been kept too long in adverse conditions or maybe they just arrived at the fish shop yesterday and were still trying to become acclimated to the shop's water. Once a fish has been in my tank for a week, I figure that any problem they have is my own fault. I failed to provide the needed water parameters or did not feed them the right foods or I even asked them to adjust to water that was too far from the water they were raised in. At under a week, I look to the fish shop as a possible problem area. Shops have the same problems as we have with water parameters. They also receive fish that must be properly acclimated and that must adjust to their tap water conditions. If they differ too much from the water their supplier is using, they will have acclimation troubles too. A fish stressed by that will never make it through the further acclimation to my water, which is different to my LFS's water.
A single large change in water parameters can be done over a few days and almost always will succeed. A double or more need for a large acclimation is another matter entirely. If your LFS does not try their best to match incoming stock with its original water conditions, the fish they sell you will start out highly stressed even before you get them. Those fish tend to perish all too easily.
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Have you tested the water in the main tank and done a water change to be on the safe side?
