The return policy for live fish at Petco is that if it dies within 30 days you can bring it back to the store, accompanied by a water sample. If the water test show that the tank is healthy they give you a new fish. This isn't complicated, but in my two years working there people found some interesting ways of messing this process up. Most commonly people would bring the dead fish in the water and when I told them I needed a fresh sample they were always furious. "I brought you the fish and I brought you a water sample! I'm not going home and coming back to get you the same water." I suppose that it didn't compute for them that a decomposing animal might affect the test results.
I could always tell when someone brought their tank water from Pawtucket because it became my job to tell them that they were putting fish into water with a pH level similar to orange juice.
The dumbest man I ever met came in with a dead fish and no water sample. When I told him he would have to go home and get some from the tank he looked at me blankly. I repeated myself, "I'm sorry sir, I just need some of the water from your tank before I can give you a new fish". He said nothing and continued to look at me as though I had an ass on my head. After I repeated myself for a third time he said, "I don't have any water." I assumed he meant he didn't have any with him so I explained the policy yet again.
After a back and forth that was felt like an eternity I got to the bottom of why his fish hadn't made it. He had returned home from the pet shop with the fish in water. He then took the fish out of the water and placed it on his table. He genuinely wasn't sure why it had died...