Sorry, I was kind of working it out in my head as I typed. Plus I was a little tired from doing the same thing, hundreds of times today, so my brain was mush. Better now.
When you dissolve a salt water molecules align themselves around the ions to stabilize them, this is called the waters of hydration. The result is that to dissolved compounds, there appears to be less solute, or, in other words, they appear more concentrated in salty water than in fresh. You can try it on a flat beer or pop, when no more bubbles come out, throw some salt in and it will start foaming again. This is because less water is available to dissolve the CO2, so it's once again supersaturated.
So if the makers of the kit prepared their standards (samples with known concentration) in salty water, which presumably they would for a marine test kit, then using it on a fresh water system would have just the opposite effect. If you could somehow magically pull all the salt out of the sample, the waters of hydration would once again be available to dissolve other stuff making the Ca seem less concentrated than it really is.
It's a tricky concept, which is why I was so confusing about it. I think that my conclusion may even have been wrong, let's find out.
You prepare a standard having 2ppm of Ca in salt water and measure the concentration. Now you remove all the salt, and it appears that there's more water present, so the concentration now reads 1ppm. The thing is, there's the same amount of Ca in the solution, just that one has more water available to the Ca, so it's like you tested your water, then took another sample from your tank, added a little RO water and then tested again.
Basically, all I'm saying is that based on this, take the reading as a concervative measure of the Ca concentration in your tank.
There might be some difference in the unit of ppm too, but I'm just thought about that, I'll post later if it's relevant. It has to do with the density of fresh vs. salty water.
