SW Nitrate test needs to be halved, I beleive:
From:
http://home.comcast.net/~tomstank/tomstank_files/page0018.htm
How the nitrate test works:
Cadmium metal (says Hach), or perhaps Hydrazine (says TheKrib.com), or maybe even Sodium Bisulfite (says the AP MSDS sheet), is used to reduce nitrates to nitrites. Nitrite ions react with sulfanilic acid to produce an intermediate diazonium salt, like in the nitrite test. The diazonium salt then forms a red-orange complex with chromotropic acid. The reaction after the cadmium, hydrazine, or bisulfite (or whatever it is!) reduction is the same as shown in nitrite testing above. Thus, the test actually measures nitrate by the amount of nitrite it can be converted into.
Interference: (not a complete list, where interference levels are given they are for the Hach Co test, and might not appy to Aquarium Pharmaceuticals)
Calcium (100ppm), Chloride (>100ppm cause low results) which means your saltwater test will yield too high a result in freshwater (SW kit calibrated higher to compensate for the lowering effect of higher chloride), Ferric ion
Nitrite – since the test measures nitrite, any nitrite present will add to the observed nitrate level. Since your nitrite should be near zero, this usually doesn’t matter. Also, since your Nitrate kit is testing for greater than 5ppm, you wouldn’t notice the nitrite effect unless it gets very high, at which point your tank is really out of whack and nitrates are your last concern. Nonetheless, if your cycle seems stalled because you have high nitrite and and nitrate that isn’t changing, perhaps the nitrate you thought you had was really mostly nitrite.
