Five years ago, Peta led a 7 month raid against Texas based exotic pet wholesaler, US Global Exotics (USGE). They found that this wholesaler was losing, on average, 3,500 animals per week, due to overcrowded, unsanitary conditions and lack of feeding. Of 24,000 animals kept there, over 80% were dying, with no chance of survival. The raid actually went down as one of the largest in U.S. history. Biologists were brought in, and were able to determine that USGE was responsible for the spread of animal-borne pathogens to homes, zoos, and pet stores.
http://news.mongabay.com/2014/1002-wilson-usge-animal-loss.html
When they began looking into who bought animals from USE, Petco was at the top of the list. Now, this doesn't pertain directly to reef aquariums, but it does reflect on where Petco gets their animals. USE was known to undercut other wholesalers, and kept far more animals then they could realistically care for. Many Petco's operate like small USE's, packing animals (reptiles, fish, etc) into overcrowded, unsanitary conditions, and allowing people with no credentials to sell them to an ignorant public. Why do they do this, money. USE's are far cheaper to buy from then wholesalers who care properly for their animals, and they don't ask any questions about how the animal will be cared for on the retail side of things.
Petco didn't get to where it is today, by selling a few well cared for fish here and there, by employees educated in reef aquariums, which were paid a wage based on their knowledge. Petco got their mounds of millions, by working with places like USGE, and using minimum wage employees, with little to no knowledge about reef aquariums, to sell livestock to anyone.