What is the ingredient in the powder?

Using bleach is easy. Your needed chlorine level is based upon your CYA level. (See the Best Guess table in my signature below for more about that.) I use bleach exclusively. Each evening I test and add a little bleach. My CYA is usually around 35-40, so I keep my chlorine between 3-6ppm based on the Best Guess chart. So, when I see what my cl reading is when I test in the evening, I add enough bleach to get back up to 6. That's all there is to it. I am diligent about testing and don't let my chlorine levels get too low. If you do, you risk an algae bloom and then have to shock up to the needed level shown in the Best Guess chart. Very easy. For reference to help you know how much bleach to add, each quart of 6% bleach will add about 0.8ppm of cl to an 18K pool. So, while your CYA is 20, you'll need to keep your chlorine between 3-5ppm all the time except when you are working to clear it. Then, your shock level will be 12ppm.

You may want to bump your CYA up just a little. 20 is pretty low. We usually suggest around 40-50. Some powdered shocks have CYA in them, thus the reason for my question about the ingredients of the powder. If what you used was dichlor, then that would have added some additional CYA. If your powder was cal-hypo, then it wouldn't have and you'll want to add a little CYA separately. We'll decide that after you tell us the ingredients.