I use AP test kits as well, from petsmart too. They work for giving you the general idea, but for accurate readings, find another test. Even PH is not always accurate, Ammonia is rarely accurate. Mine shows .25 ammonia as a minimum. And if memory serves correctly, there saltwater ammonia test is the same as teh freshwater test, I've compare the two. 0 on freshwater, .25 on salt. Just inaccuracies in color charts that's all. But I've had my current display tank for about 2 years up and runnin and only used the AP test kits, I still get by just fine in the hobby. Although there have been a few times more accuracy would have made a big difference for me. Like wasting a bajillion ammonia tests trying to figure out what was causing a problem that wasn't even there.