how much soda ash or 20 Mule team borax do I add per gallon of water to raise my SPA's pH level?

paul e asked:


the spa was crystal clear when uncovered then after being exposed to sun it turned clear yellow (no one was in it). I measured the pH and it was very very low. I saw that you could substitute 20 Mule Team Borax for pH up – but do not know how much to add per gallon
the spa is 280 gallons

Day Spa Alpharetta