The main supplies I buy from PetCo are for my saltwater aquarium. If you have a dog, cat, or even a horse, they have a good selection of stuff, but for a saltwater aquarium, it can be slim pickings. They have recently upgraded the marine section, so it is better than it used to be. I don't buy saltwater fish here, because they don't care for them properly. The tangs are in 20 gallon tanks (they need at least a 30-40 gallon tank when they are small), the dragonettes shouldn't be sold to just anyone, and none of the fish have anywhere to hide, which leads to stress, which leads to ick.
If you have or are thinking of getting a saltwater aquarium, I would recommend finding a specialty saltwater store for the livestock. Supplies are usually cheaper at PetCo, if they have what you need.