First problem is cost and space (where to put all these QTs?) QT'ing fish means you can't sell them right away and must spend $$$ on tanks, salt, equipment, medication, pay employees to maintain these tanks, etc. You would have to pass on that cost to your customers with higher prices and that's just not gonna fly.
Second problem is you'd need employees trained and knowledgeable about treating fish diseases. Copper isn't a fix all. And if you don't know what you are doing, you can actually kill fish with copper by overdosing.
Thirdly, they don't care if one dept. is losing money. It's all about the overall financial success of the store. So long as people are paying top dollar for dog food, grooming, supplies, equipment, etc. then it doesn't really matter if the SW fish dept. is losing a little money. Seeing the colorful fish brings people into the store.