api is def the worst for calcium they dont do much for the big3 , just the basics nitrate,nitrite and amonia....anything else and i would use a different test
cheers
carefull with the purple up it can increase your calcium by up to 40ppm per dosing , and its possible to over dose so stick with the guidelines
i use purple up with iron and iodine as a suppliment. i use it very rarely though:P