What is your stabilizer (CYA) level? And do you have a combined chlorine level, or just total chlorine? Are you losing chlorine at night (which would indicate algae or something else organic is using up the chlorine) or is it staying stable at night and disappearing during the day (indicating the sun is depleting it because your stabilizer level is either not high enough or your shock level isn't high enough). If you could please give test results for FC, CC, pH, and CYA, it would really help us to help you.

Assuming your CYA level and you're shocking to the correct level, JohnT's advice is right on the money--when you're fighting algae or other organics, you need to test and add chlorine to get back up to shock level at least 2-3 times daily--the more consistently you can maintain your shock level, the faster the problem will clear up.

Janet