You need budblood the first week of flower and then the additive bigbud for the rest of flower.

When you run a rez of any size your plants drink water. As they do your nutes PPM will rise. You add back to the rez Ph'ed water to keep it from getting too strong and burning your plants. If you have a 5 gallon rez and over a weeks time your plants drink 5 gallons you are ready to change the rez. You would of replaced the 5 gallons topping off each day.