ironic, i just picked up a bottle of that stuff for the first time!
heres an exerp:
"COMPATIBILITIES
AmQuel+ is harmless to plants, fishes, invertebrates (including corals and sea anemones), and other aquatic life forms. This product is safe for use in aquariums and ponds and will not interfere with biological filtration or its bacteria. When AmQuel+ detoxifies ammonia, nitrites, nitrates, and other organic compounds, it breaks the links of molecules apart. The nitrifying bacteria do not know the difference and eat the parts, just as they would the whole original compounds.
Where AmQuel+ is particularly useful is in its detoxification of all nitrates in the water. There are not sufficient bacteria in the aquarium (fresh or salt water) and pond water to do this. In the past the only way to get rid of nitrates from aquariums and ponds was by water changes. Now this can be done in a few minutes with AmQuel+. This leaves the main reason for making water changes (now needed less frequently) to replacing trace elements in the water. For this Kordon's fresh or saltwater VitaTrace can be used that replenishes both the trace elements and vitamins needed in the water."
So yes, it is perfectly fine to add the amquel+ in with the tank fully set up and stocked.