Are we talking about the API ammonia kit? If so, see this thread with a poll by
@MnFish1 :
There is widespread commentary out there that certain (API) test kits for ammonia are notoriously inaccurate. 1. I thought it would be interesting to see how common this thought is, in general - and compared to other tests. 2. At the start I will list several testing errors that I have seen...
www.reef2reef.com
The poll shows that there 4x as many people who think the kit shows false positive then people who have actually used the kit and had a problem.
Post #4 sums up my thoughts on the API ammonia kit.
"The kit has one issue, and that issue is that zero can look like 0.25 ppm to some of our eyes.
It drives me bonkers when someone posts a 1ppm reading with this kit and people say it's a false positive because this kit gives high readings.
The kit works fine. I can test freshly made saltwater, and it reads zero. I can test my cycled tanks, and it reads zero. I can calculate how many mLs it will take to cycle my quarantine tank, and the reading will be close after I add the ammonia. People who say this kit doesn't work either haven't used it, don't know what they are talking about, or both."