API Ca++ Test Kit Issue

[BrewinProf]

Hello, I'm having an issue with the API Ca++ test for a relatively new reef tank. This kit involves adding one drop at a time of the second reagent until the color changes blue to indicate total Ca++ level. The problem I'm having is that there is no sudden change of color, rather the color change to blue seems to be very gradual starting at about 380 ppm, which continues to intensify until well past 460 ppm. I haven't gone past that point but even at 460 ppm it still doesn't completely match the color standard shown on the instructions. I've used this test for a previous reef tank and didn't have this issue, so I'm wondering if it is a defective kit or perhaps some other water parameter is interfering with the test. My other parameters that I've measured are Salinity 1.025, pH 8.1, KH 9, Mg++ 1350. I've emailed API tech support but they never got back to me. Any thoughts? Thanks!

 

[Randy Holmes-Farley Verified Community Expert]

I've used this test for a previous reef tank and didn't have this issue, so I'm wondering if it is a defective kit or perhaps some other water parameter is interfering with the test.

What color change over what range did you observe on the other tank?

 

[BrewinProf]

What color change over what range did you observe on the other tank?

Thanks for the response. It's been about 5 years since I had my last tank but from what I remember on my previous tank it was a sudden change with a single drop or two. Similar to how the API test for hardness works. Of course this is a new test kit, i.e. I'm not trying to use a 5 year old test kit now.