Chlorine Demand

[Watermom]

To add to the advice that Janet gave you above, your pH is a little low. Use some 20 Mule Team Borax (laundry aisle at Walmart) to bump it up a little. Aim for 7.4-7.6 but anywhere 7.2-7.8 is ok. Start by adding a half a box slowly to the skimmer while the pump is running. Retest after a couple of hours. NOTE --- You cannot get an accurate pH reading when your chlorine level is high. Adjust your pH a little first because with a chlorine level of 20, you won't be able to get a true pH reading.

[gfritz1]

Can I use Lithium shock as have a pile of it?

[PoolDoc]

I don't think it will hurt anything to use it, and it will add chlorine, so yes, go ahead.