I learned a long time ago that one is gambling big time when buying fish from box stores be they full scale department type stores or pet store chains.
We are all wired to want to pay less for things and to seek out bargains. This does not apply to buying living things. The problem is magnified when it comes to fish. It is difficult to know the condition of many fish when we get them. If they have no clear external symptoms, if they seem to be doing OK in the store tank we tend to buy. Often problems get chalked up to store tanks being over crowded.
Most stores seek to turn over fish as quickly as possible. They also dislike having to feed them. So they have fish that have journeyed from the wild or from fish farms, often stopping along the way briefly at a wholesaler. The fish are normally starved for a day or two prior to shipping each time as well. So by the tie they reach the store they have been stressed and barely fed. And then the come home into our tanks for one last shot at being stressed. Is it any surprise that many stores are selling fish that may not last?
I have a bit of experience with the sales side of things as I am able to buy some at wholesale or import prices. I know what store markups look like. The difference between what the fish cost when they first arrive in country vs what they ultimately sell for is 3oo - 500% in most cases. This is especially true for the least expensive fish. Bear in mind that those 6 tetras you get are normally sold in a box lot when they first enter the system. That means the first buyer may be receiving from 150 to 300 or 400 fish small corys or rasboras etc. Big sellers get them by the 1,000s.
I buy almost no fish from brick and mortar stores and haven't for a long time. I deal with smaller scale importers whom I know will quarantine fish and medicate them as needed. I buy from breeders when possible too. I am fortunate in that I am a fish nut with 15- 20 tanks running for over a dozen years and I learned who some of the good sellers are. For the average hobbyist the choices are often limited by what is available in their area or whom they know in the hobby. If you have a decent LFS (not LPS) within range, if their fish are healthy and their staff experienced in keeping fish, most people are better off paying them more money and ending up with fish that start off in good shape. That way when they go into appropriate new homes they should do well.
In the end, one live healthy fish has to be worth at least three dead ones.