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My personal opinion is it's a nice visual indicator if using during cycling and testing regularly as well, but in a fully cycled system, it's totally useless. If your system is cycled, there should be no ammonia. So to me it would be a waste of money.
I agree that it is a waste of money. If the tank is cycled you shouldn't have ammonia spikes unless the bacteria is killed of. Also, if you use prime or any ammonia detoxifier it will alert to ammonia even when it is fine because the test can't distinguish that the ammonia is ok.