Ammonia alerts measure toxic free ammonia rather than TAN. They are a good visual for tanks that are expecting ammonia spikes and should be used in conjunction with the API test kit in my opinion.
When I did my fish in cycle I used one of these. I tested for ammonia every now and then. Ammonia alert stayed in the safe zone and API never went above 0.25. I have nothing bad to say about the ammonia alert other than be aware that they are not measuring total ammonia. This may be one of the reasons people report such large discrepancies if they do not understand how it differs.
Edit: and that wasn't a swipe at you Laeris
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