Watermom has told you about the CYA; let me tell you about chlorine and algae: do not let it be zero any more.

THIS evening, withing 2 hours of sunset, add 8 gallons of plain 6% household bleach. Check your chlorine the next morning within 2 hours of sunrise. If your chlorine level is less than 5 ppm, repeat evening doses . . . and continue that till you can add at night, and find what you added still there in the morning. If you've got algae growing visibly, brush it AFTER adding the bleach, and then add 2 more gallons.

What I'm getting at is that there two separate issues. One is that you are losing chlorine during the day, to sunlight. The second is that you may be losing chlorine ALL the time to algae or 'goop' in your pool. Adding chlorine at night and checking in the morning tests to see if you have a high 'chlorine demand'. If so, that ALSO needs to be fixed.