Wow... I had it in my 55g cichlid tank, and the sensor read 0 since day 1.
But I stuck the tester in my 10g feeder tank, and within 15 mins it turned to the "Alert" level (the 2nd lowest level after "Safe".
Then I put it back in my 55g again. Within a half hour it went back to the "Safe" reading. Then I put it back into my 10g again and it went back to "Alert".
so, IT WORKS!
time for a water change in the 10g now
for a mere 7 bucks, I am happy. And to think, all the more conventional, manual forms of ammonia testing cost at least 9 or 10 bucks. Woohoo.