To be fair - there are Petco employees who care for and respect livestock as much or more than anyone else. I've known MANY great employees of all the various chain stores; those who had limited other options for working within the industry, those who needed a job with flexible part-time hours for school, and those who wanted to try to fix the problem from the inside, rather than standing on the outside and complaining. Here in central Indiana, many chain store employees were responsible for documenting and providing evidence that shut down a huge portion of the state's puppy-mill traffic.
Bear in mind that the manager who wouldn't sell you that fish is working within a restrictive corporate structure. Here's an example of why he may not have been willing to discount... Some of the chain stores, depending on who the management is, use an ordering algorithm. Each time a fish is brought in, its performance is logged. They see whether it sells fast, if it sells for full price, if it lives, if it is claimed against a DOA policy... all of those metrics are cataloged for a few months, and the computer looks at the numbers before generating a report - what fish sell for profit, and which are losers. After the report, only the profitable fish are allowed! The ordering system doesn't allow for non-profitable items to come in, or if it does, they require direct approval from someone in upper management so they can override the block. There are a few things that keep a fish from being recorded as a loser, and claiming them as DOA or diseased is one of them. By having the corporate 'expert' come in, they can get that fish signed off as 'sick upon arrival' and the blame for the loss isn't assigned to the store. If he had sold you the fish at half price, Purple Tangs would have a mark against them in his ordering system. If he sold both that way, it would demonstrate a downward trend in Purple Tang pricing. Considering that Purple Tangs are only seasonally available, and considering it's Petco, they probably don't stock a ton of them, so having 2 losses could affect the whole year's average for Purple Tangs, meaning the next year they're not allowed by the system. In this case, the manager was only protecting his ability to continue to offer that stock at his location.
I'm not justifying the loss of those 2 tangs... I feel that every store selling any sort of creature should have Quarantine capabilities for those creatures, so that they can be treated humanely for whatever illness they get. It's a responsibility inherent to the trade in live organisms that's too overlooked. When the sick creature is mixed in with healthy stock, it's easy to justify "culling" it to preserve the health of the majority and protect the investment. Quarantine removes the financial incentive to euthanize and encourages more ethical veterinary practices.