I just switched to the Red Sea Pro kits for KH, Ca, Mg, N-NO3, P-PO4. I like them better than the API I started with and the Salifert I went to next. I'm still using the Salifert kit for double checking Mg since the Red Sea kit seems to test high if you don't use it exactly per the directions. Watch the videos on the Red Sea site or YouTube to see how its to be done.