The chlorine that you put in in the evening is working more efficiently than what's put in during the day, because the sun is helping consume the daytime additions....but that being said, the more consistent you are about keeping it at shock level, the quicker the algae will die. If you are home and able, then by all means add Cl during the day, too....but if you're not home, then morning and evening will also work, it may just take a little longer.