For your stabilizer to have dropped from the 30 you dosed to, to 20 would need for you to have lost and replaced 1/3 of your water. Your likely still in the high 20s so I wouldn't add more just yet. Keep dosing to 12ppm FC
This is the hard part now. Your chlorine held overnight which means your algae is mostly dead. Now it's just a matter of allowing the sand filter to clear it up. Continue to brush, and maintain your chlorine level at shock level until you can clearly see the bottom and you have 0.5 or less CC.
To help your filter you can add a small amount of DE (diatomecious earth). To do this, backwash and rinse it well and look at your pressure. Then add a tiny bit mixed with a bucket of water(1/4 cup de, maybe less). Then check the pressure again after waiting two minutes. Your looking for a rise of 1psi and not more than 2psi. If it didn't go up by 1, add a little more until it does. This will improve your filter efficiency and you will likely notice your pressure increases much faster and you will need to backwash soon. Backwashing will also removes the DE, so you will have to add more after each backwash. ( If you overshoot when dosing, you can also backwash to restart). This is optional, but otherwis,e sand filters can take a long time to clear up dead algae.