With deco, substrate, and anything else that occupies volume in the tank, it is pretty much impossible to know exactly how much water you are actually adding to the tank (when using a hose).Why dose for the entire tank amount if the water in there has already been dechlorinated? Then you can dose for what you are putting in and save money. Also wouldn't it over dose and possibly cause bad reactions?
Prime can safely be dosed up to 5X the manufacturer's recommendation...better safe than sorry
I've used Prime for the nearly 10 years I have been keeping fish; I've never had a single negative issue with it...but I may look into the API equivalent when I need to get more, just because of what I've read from others here about it's effectiveness, and the lower dosing recommendation (I'm always willing to save $)