It is almost impossible for a LFS to properly quarantine due to livestock turnover. If the store is doing any kind of business, most fish will not be on the premises long enough to have completed a meaningful quarantine. To do it right, they will have a hard time staying in business.
That is why a smart consumer always does their own quarantine, as you are planning.
It is probably more important that they are selective about their suppliers. As where they get their livestock and look up those suppliers.