OK,
If you haven't fixed it yet:
How many gallons is your pool and how low did pH go? Is your pool vinyl or gunite/concrete/tile?

I'm guessing here on what's best, and with that much acid I'm guessing it's WAY down. I rarely even add a quart of acid to a 20,000 gallon pool:

If your pH is the lowest on the scale, dump a box of 20 Mule Team Borax in IMMEDIATELY into the skimmer. This is especially important if you have a vinyl pool. Keep the pump and filter running constantly (high speed, if it's a 2 speed pump). Wait a 2-4 hours and test pH again. If it's STILL off the scale, add another box and wait 2-4 hours again.

Keep doing this until you get a pH reading that's on the scale, even if it's low. With a vinyl pool, you want that pH to be 6.9 or better or you risk damaging the liner if you don't get pH up in a reasonable amount of time.

Once it's 6.9 or better you can relax. If your pool is big (30K gal or more) you may want to add another box of Borax. If it's smaller, (20k or less), you may only want to add 1/2 box. If it's really a small pool, only 1/4 to 1/3 of a box. Wait, re-test.

Thankfully, 20 Mule Team Borax is cheap and at most supermarkets. It's also really safe and effective.