The chlorine was below 10 when you tested PH at 7.8 so it is a good number. Enter your pool gallons in PoolMath, enter your TA number in now and target and enter PH now 7.8 and target 7.2. Add the amount of acid it says to add. Make sure you have the correct acid percentage for what you have.
The PH test is not valid when chlorine is above 10.
The PH test is not valid when chlorine is above 10.