I have used with great success for many years...
- API for calcium, fast and easy to use. Also consistent and around $12
- API for alk, VERY fast to use. Very consistent and around $6-$7
- Salifert for mag, very consistent, not that fast to use. Generally around $35-$45
In the very rare occasion I have to check for phosphates or nitrates, I use API. Not decimal point accuracy but accurate enough that I know if I have a problem or not.
No idea why the above said to stay away from API. Years of successful SPS farming and beautiful tanks say otherwise.