I have an API, it's garbage IMO. Compared the results to Salifert and Tetra, and a redsea, the results are always off, showing considerably lower values.
Tetra seems to be the most consistent for the $, my lfs swears by the brand. Whatever you decide, stability is the most important parameter to keep. Stuff has thrived in foul water, only to die when the water problems are corrected rapidly. Unfortunately I've had this happen on a tank given to me by my eldest kid, the water was totally screwed, 50% water changes twice a week over a couple months killed a breeding pair of clowns, a firefish, some damsels..