In a 20,000 gallon pool, it will take approximately 16 oz. ( a pint) of muriatic acid to drop the pH by 0.2. But, we have learned not to try to adjust something all in one dose, right? So, we want to do this gradually. I think I would add 2 pints. That should take you down to 7.8 but lets see what it does.

Muriatic acid is nasty stuff and you need to be careful with it. Pour it slowly and carefully in front of a return jet. Be careful not to splash it on yourself or on the liner. Try to stay upwind of the fumes and it is best to wear gloves and eye protection. Keep the pump running to let it circulate.

This is also going to drop your alk. So, you're going to need to add some baking soda to raise it. A good range for alk is 80-120.

You also need to add some bleach. With a cya of 25, you want to keep the cl between 2-5 all the time.

What order to do these things? I would add some bleach first. Actually it would be ok to add a couple of lbs. of baking soda at the same time as the bleach. After circulating for a few hours, then add the acid. Then, wait 3 or 4 hours and retest and then go from there.

EDIT -- Just reread your post. Did you mean that your chlorine was >5ppm instead of >.5ppm by any chance? If that is the case, you don't need to add any bleach right now.