I asked the same question a few weeks back and the general consensus is that the API test kits are just as accurate as any other on the market but they are not as precise. The calcium test goes by increments of 20 points where the Salifert is precise to 5 points.

Is API nitrate test accurate?

Both api and nitrate-nitrogen test kits are accurate. Just calibrated to measure different readings. There are 4+ some change total nitrates for each nitrate nitrogen.

Is API pH test accurate?

Use the API® pH TEST STRIPS as a quick and accurate way to test pH levels in your water. These test strips measure a wide range of pH, from 6.0 to 9.0 ppm, to help you keep aquarium conditions safe for your fish.