Hey Fishlvr13! Welcome to the forum!
You may be looking at nitrites a little bit differently than I might...your goal here is not really to try to reduce nitrites as much as it is to 'grow' and protect beneficial bacteria which will 'eat' or oxidize nitrites into nitrates. That bacteria is what API purportedly has placed in the bottle of stress-zyme that you bought...so you really cannot expect instant results from using it.
The major method in which nitrites become harmful to your fish is that they inhibit a fish's ability to 'breathe' by making their gills stick together, more or less anyways. You can help your fish to some extent by using a bit of aquarium salt or table salt (not marine salt). I suggest starting off by adding one tablespoon or so of salt...sodium chloride to be exact...per every 10 gallons of water in the tank, keeping in mind that your tank probably doesn't hold as much water as advertised since substrates, decorations, and fish themselves displace some of that water volume. Anyways, be sure that you dissolve the salt in a cup of tank water and then add it slowly so as not to shock your fish in any way. This should help to some extent when your nitrite levels are near or less than 1 ppm. At higher levels (I believe that 1.5 ppm or above is usually a recommended 'threshold') doing a large water change is your best bet....probably closer to 50% versus 15%.
Of course, the other option would be to remove all of the fish and inverts from the tank, do a water change, and enjoy low nitrite levels forever...but then again, what fun would that be???
(Ok, bad joke, sorry)
