Check your TA. If it's 0 (immediately red), then the pH is 4.5 or lower and you can use pH Up (or Arm & Hammer Super Washing Soda) to raise the pH since you'll need to replenish carbonates (TA) in that case anyway. If you have TA, then using the Borax is fine for raising the pH, especially if your TA is in the ballpark (say, above 60 ppm).