I see this thread is a little old, but I figured I might be able to provide some information here. For several years I was the general manager of one of the largest aquatic livestock wholesale suppliers in the country. Wal Mart was one of our large customers and we serviced hundreds of their stores.
As others have mentioned, buying fish at Wal Mart is highly dependent on the quality of care they receive at that particular store. Some pet department managers were very conscientious and took great care of the tanks, but sadly this was the exception to the rule. In reality, most pet department heads were so busy doing their other chores that they weren't afforded enough time to do the basic tank maintenance that would be required to keep the fish healthy. Furthermore, many store managers felt that the tanks were a waste of time and actively discouraged their employees from working on them. This was frustrating as a wholesale supplier because we were required to give Wal Mart a very generous credit policy for any fish that died. Store management would often abuse this privilege. It was easy to tell which stores were doing the basic maintenance and which weren't simply by looking at the credits. And my personal on site inspections backed up this conclusion. As a vendor, we were not given much support from Wal Mart home office, instead we were told that we had to accept the poor performing stores if we wanted to keep the few good ones that we had.
Furthermore, I saw outright sabotage in many instances. In one store, the tanks were back to back with metal shelving that held lawn and garden chemicals. Bags of fertilizer and weed killer were stacked on these shelves, and product from ripped bags was falling into the filter sumps housed below the tanks. When we inspected this store to determine what was causing the high fish mortality we asked management to move the chemicals so they weren't adjacent to the fish tanks. At first they promised to do this, but they never did. I was told that the management in that store, and the district manager in that area did not want fish tanks in the store, so they hoped that their poor sales would justify their eventual removal from the store. At another store, contaminants were being placed in the tanks. We would flush the system, only for the problems to return. I once spent one full week at that store, helping in the fish department trying to get them back on track. The day after I left contaminants were again placed in the tanks.
As for the quality of the fish that Wal Mart receives, well, it was the same fish that we were selling to pet stores. The fish were mostly flown in from Asia each weekend where they would sit in tanks in our fishroom as we sold them. For most fish, especially tropicals, we would turn them over on a weekly basis. Pond fish and certain goldfish were sourced locally and might reside in our tanks for longer periods. But in all cases, we sold the same fish to Wal Mart as we sold to any other customer. What happens after the customer receives them is a different story.
It is not all doom and gloom. There were several Wal Marts that had bright pet department managers who took a genuine interest in the fish. The tanks were all modular and connected to a central filtration system that required very little maintenance. In these cases where an employee was willing and able to do the basic maintenance, and stay on top of any problems that might arise, the fish were in excellent health. So it is possible to get good fish at Wal Mart. You just have to do your homework and find out if a store near you is doing a good job or not.
Better yet, I personally would do business with a local aquarium or pet store if possible. I think that in general the care is better because these stores are often run by the owner. Plus, its good to give business to small local independent business owners, if for no other reason that to encourage diversity. For example, we were only permitted to sell to Wal Mart a preapproved list of a few dozen fish. Pet stores on the other hand could order virtually anything from us.
And finally, no matter where you get your fish or plants, it is vital to quarantine them before you put them in your show tank or pond. Most of the fish you see at any pet store are only days or weeks removed from huge breeding facilities. Plus, they have travelled hundreds or thousands of miles to get to your local store. Fish disease is a fact of life under these conditions. It is much easier to recognize and treat a disease when the fish is in a small observation quarantine tank. Plus, you don't run as large a risk of infecting your other livestock with the new introduction.