We have been having problems with our pool for a while and finally seemed to get them under control. But today I tested and it seems our pH is low! I read where you can add Borax to the skimmer to increase it - does anyone know if this is true??? Also, we have a pool cover and a pool heater. I live in FL and the days have been mid 70s lately falling into 60s at night. Though this past weekend it fell into the 40s at night...could my pool cover have an effect on the pH falling?

In other words...what should I do??? I am having friends over later this week and we'd like to swim but I want to make sure everything is balanced...