me personally, i would make it a priority to get to an LFS that is not petco, to get some nitrifying bacteria. if you havent already, i would be doing it immediately as soon as they open. this will help eliminate the ammonia issue, but water changes will be needed. everything that jfoahs has mentioned, is the advice that most, if not all, would go with. that water from petco, should be ok, but youre throwing money away. you would be best mixing your own RODI saltwater or buying it from the LFS that he mentioned. until you get comfortable with things, i would probably steer clear of petco for now. its not that they dont have good products, but they are not giving the best info, or are just selling things just to sell them, expecting the customer to know what they need. get yourself started with the LFS mentioned, and then when you need little things, petco might be an option. i personally try to give my business to an LFS anyway, unless they dont have what i need. Salt is the one thing i use a large retailer for, and thats because
chewy sells buckets of instant ocean salt for half the price of anywhere else.
i hate to be the person that says this again, if its been said already, but please do some reviews, check out youtube videos, or whatever, to get some beginner direction on how all of this works. that way, you know the why behind everything that is being suggested. these forums are a great place to start. any question you have, or question you dont even know you have, has been asked here at least a dozen times. its a great place for help, so get yourself going on how saltwater aquariums, and aquariums in general need to work, to keep your finned friends alive.