This is a discussion on Do these things even work? within the Freshwater and Tropical Fish forums, part of the Freshwater Fish and Aquariums category; -->
Amazon.com: Seachem Ammonia Alert: Pet Supplies
Amazon.com: Seachem Ammonia Alert: Pet Supplies...
They work, but by the time you are in the danger zone, the fish may have died! I used one when monitoring a tank having cycling problems just to give me a visual clue regarding the tank's status. It was faster to leave it in the tank and visually check it multiple times/day than it was to run the API liquid test for ammonia multiple times per day. NOT super-precise, but a good trend indicator.