How can you tell if this bad every time I take a reading it is 2.50 did go to 1.53 when I did a water change for one day then back up to 2.50. Any Ideas thx.
Try a different kit. The numbers probably won't match exactly, but if they are also always the same with the other kit, then it's probably working properly.
You can purchase calibration fluid to make sure your checker is working properly. The fluid is calibrated to a known test result, i.e. 100 ppb, so if your checker reads within the test error ranges, your checker is fine.