The idea behind testing to see if you have a chlorine demand is that chlorine is consumed by two things: sunlight and pool goo. You can't really judge by looking at a 24 hour chlorine consumption whether you still have algae, because you don't know how much of the chlorine was consumed by the sun and how much by the goo. That's why you need to test at sundown after the sun is off the pool and again in the morning before the sun hits the pool if you're trying to establish whether your algae is all dead. You are always going to lose some chlorine during the day, due to the sun.
With a CYA at 100, you still need to keep your chlorine at a minimum of 8 ppm, otherwise you're inviting algae to start. You'll need to look at your results taken at night and in the morning to determine whether you still have an algae problem or just dead algae that needs to be filtered out.
If your chlorine is at 2, you need to add more bleach to get up to at least 8 ppm--but if you determine that you still have overnight chlorine consumption, then you need to shock it up to 25 ppm.
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