From my understanding from googling in the past, the reagents of the nitrogen tests are the same. The color charts are different for fw and sw, so interpreting the results of brackish would be hard.
Maybe you could take some freshwater of a known nitrate level, make the sg 1.0135, and then test again to find what color you're looking for
I took some FW from my tank and brought it to SG of 1.0135. The ph, nitrite and nitrate showed the same color as FW. The ammonia test for brackish was turbid but the color is the same. So it looks like I can use the FW kit.
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