Depending on the fish you have, I personally would not dose only the volume of water being put in, unless you are pre-treating the water prior to it going into the tank. When adding water directly to the tank, I always dose for the full volume of the tank, and turn off the filters. If you don't you will risk, chlorine/chloramine shock, as well as a possible mini cycle. Prime is already very cheap. If you don't have enough for that, either you have a 500+ gallon tank, or you are in the wrong hobby.