Ahhh the recurring Petco vs. PetNO debate. :mrgreen:
It all depends on the individual store. Some are good, some are horrible. There are half a dozen Petcos in the city where I live, and I would never buy fish from some, but am perfectly comfortable buying fish from others. Like any store, you are looking for clean tanks and healthy, active, disease-free fish. You can find that at some Petcos, and some local fish stores are lacking that too.
I started my first saltwater tank in 2000. I bought almost all of my first fish at Petco. I recently sold my tank and all my livestock a few months ago. The livestock I sold included a blue damsel, a yellow tail damsel, a sixline wrasse, a coral beauty and two pajama cardinals that I bought from Petco. Back in 2000 to 2002. When I had my first saltwater tank. The livestock I sold also included half a dozen other fish that I had purchased over the years that came from local fish stores. Was there any difference in the quality of livestock? Nope -- fish from both places were clearly healthy and grew old in my tank.
Check out the store first, before you make any judgments about the knowledge of their associates or the quality of their livestock.
That said, Petco is the ONLY store I have ever been in where I have overheard something as crazy as, "A yellow tang can be acclimated to freshwater if you give it enough time." :lol: That is a Petco associate I would avoid!