Blue water but still using up too much chlorine

[chem geek]

So you're losing 2.6 ppm out of 16 ppm FC overnight. That's a little high. If your 24 hour loss is still around 8 ppm FC, then that would be an additional 5.4 ppm FC loss during the day which wouldn't be that much out of line in strong sunlight. So it seems you have some sort of persistent loss from chlorine fighting something in the pool.

If you want to see if you can try and kill off whatever it is a bit faster, you could target 20 ppm FC as your sustained shock level. Keep track of the loss and see if it diminishes over time (especially the overnight loss). If it does, then you're on the right track. If not, then there is something else going on such as something getting introduced into the pool (such as pollen or other organic matter).

Be sure and check your filter since perhaps organic matter (including dead algae) has accumulated there and chlorine may be getting used up breaking it down. You should clean your filter after fighting an algae outbreak. Of course, check your skimmer as well, clean out your pool sweep bag, etc.

There's one other possibility you should eliminate -- the chlorine may not be as strong as you think. If you are assuming it's 16 ppm FC you are getting to at night based on the quantity of chlorine, you should instead wait at least a half hour (with good circulation; an hour if poor) and test the FC level, then test again the next morning. It's possible the chlorine you are using is weaker than you are assuming.

Richard