Well, data can often confuse one
First, there is NSW and then there is NSW. You need to look at the source. Where was the data taken from and by what method. What was the salinity, the depth, temperature, how far off shore. Any NSW sample could be called NSW but may are skewed way off from what you think, because of what I said above. When most of us talk in terms of NSW we mean the Std NSW, often found in chemical oceanographic books and on some oceanographic sites. Mg / l is not the exact same as PPM. Mg / l = ppm x density (no, not specific gravity the are not the same thing ). For us that would be a density of about 1.023, so 400 ppm Ca x 1.023 = 410 mg / l Ca but they are close enough for us so we say they are the same. The NSW Std is referred to as Copenhagen Seawater. You can actually buy this Std from labs and Copper is 8 x 10^-8 M / l x 63.546MW = .05 ppm =.051 mg / l or about 50 ppb.
Then there is the Cu ++ in ASW, another matter. Each salt type and even batch can have different readings. In the past, years ago, when tested, the range was from . 1 - .2 ppm Cu++. The new report by Timothy A. Hovanec, showed all salts to be ND (non-detectable, based on their equipment). However, Ronald L. Shimek in his test, just prior, showed it to still be present by less than years ago .01-.1 ppm. Std treatment for Ick is .15-.25 ppm
Most cooper that enters seawater in tanks leaves quickly. It is easily picked up by carbonate substrates, the reason behind a bare-bottom tank when treating for ick.
Samo
When the reagents starts to go bad on the SeaTest kit it often gives false readings of Cu being present when it is not. Try a test on a fresh sea mix and one on just Ro/DI water. If they all read the same the reagent is bad
