Here is the method that was posted long ago by another mod, CarlD, for checking the strength of your chlorine..
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Put ten liters of tap water ( That's 5 soda-bottles full) in a 5 gallon bucket.

Test that water for chlorine using the FAS-DPD test to get a chlorine baseline (you may have some).

Then I take a 1 ml eyedropper full of the LC or bleach I'm testing and put it in the bucket and mix it up.

(1 ml in 10 liters of water is the same as putting 1 gallon of LC in a 10,000 gallon pool -- it's 1/10,000th. )

I then measure the bucket with the FAS-DPD test just like pool water. Of course, subtract any chlorine you measured in the tap water.

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One gallon of 12.5% chlorine should add 5.4 ppm of chlorine to 23K gallons. It is possible that the chlorine is being eaten up by algae, but not at that rate. 10 gallons should add around 54 ppm, and if you can't see visible algae in the water, there's no way it's eating up 49 ppm of chlorine before you can measure it. Any chance there's ammonia in the water, or hvae you added polyquat or something else that can create such a big demand?