Wow, never heard one go as far as to say they were a "specialist". Must either have a lack of shame or is extremely blind to the fact he knows nothing.
Water paremeters are, in comparison, a toxic gas. NitrAte isn't very toxic and only seriously affects fish at 40ppm+, although the ideal amount is 5-15ppm. Ammonia is the most serious "gas". It burns the gills of fish and can lead to internal damage, and regular esposure almost always ends in death. You should never have any ammonia after cycling, and if you see any (I usually test my 75g every water change, which is every 2 weeks. How big is your tank?), do a WC immediately. NitrItes are similar to Ammonia in affect. Burnt gills usually leading to gasping, internal damage, etc. It should always be stable.
Did you cycle your tank? Or did the "specialist" at Petco say to run it for a day, just add all your fish, or give you a bottle of cycle (AKA junk) to dump in? If you don't cycle your tank, your fish will be suffering high ammonia and nitrItes for at least 2 weeks, for me usually 3 to cycle, until nitrAtes show up. They will continue to suffer if you don't water change after nitrAtes show up. You will have inadvertanly done a "fishy" cycle, which is cycling a tank with fish waste matter to produce ammonia (which produces trItes which then makes trAtes) and doing constant WCs and testing (although it's obvious that part was left out) instead of using a prawn or fish food, which is easier on both aquarist and fish. Basically, the cycle of a tank is a deadly period of time where toxins are leveled. They require something to start on, such as fish food. Then other toxins are produced, and still others. Then you change the water 2-3 times, and add some fish (Not all, or your bacteria will be overwhelmed, AKA won't keep up with the ammonia (toxin)).
Oh, and what sized tank, how many and what kind of africans (hopefully not "assorted"), and goldfish are coldwater, so they cannot be kept with africans. I'm surprised it's still alive with them. The pleco will get 16-20" and make a lot of waste, so a 150g minimum is needed.