The more expensive the kit the more accurate it is. The best ones are hach kits but these are very expensive. API ones are relatively cheap while being more accurate than most.
well i have no better equipment to test it against, i just have to take its word that its good, as long as you are doing regular water changes it should be fine......should!
but ive found the NH3 and NO2 a little conspicuous. compared to saliferts, i would go salifert all the time lol.
theyre pretty good for your average tank but when it comes to saltwater tanks and sensitive stock i find a more accurate brand that works for you