Either way will work. If you are using a nutrient for the first time and are not familiar with how it is going to affect your pH, you should mix your nutrient first. Let the solution sit for about 15 – 20 minutes and then check the pH and adjust accordingly. Keep track of how much pH Up or Down you add to the solution. Typically pH Up will be required if you are using Reverse Osmosis filtered water. If you are using unfiltered municipal water or well water with a pH of 7.5 or above, you might need to use a small amount of pH Down or possibly none at all. Once you know how much pH adjusting solution you use in your reservoir, you can add it first of you like. Remember that temperature affects pH so make sure your water temperature is consistent when mixing your nutrient solution.