Cycling a tank with fish in it.
If you are cycling your aquarium with fish, adding Amquel can detoxify the ammonia for your fish, if it reaches levels that would stress your fish. Be sure you don't overfeed during this time. Uneaten food can create an ammonia "spike", which can permanently damage their gills, if your fish manage to survive it. Ammonia will make fish itch, gasp at the top of the water, show inflamed, red gill tissue, and can cause septicemia (red streaks in their fins) along with numerous other systemic problems. Be sure to keep track of your levels! Water changes of 30% will help keep your fish from being as damaged by the levels of ammonia and nitrite, or help you get levels under control if you've discovered you added too much ammonia by accident with the fishless method. If you are doing a fishless cycle, I definitely have to recommend not using Amquel, amrid, or any other additive that is going to interrupt the ammonia availability. You'll risk slowing down the process.
Adding salt at a dosage of 1 teaspoon per gallon to tanks can help your fish handle high nitrite levels. Adding power heads or air stones can also be a very good idea.