In a 15K gallon pool, a 1.35 dose of bleach will add 5.4ppm of chlorine. So, your addition added at 7:20 pm should have taken your cl up to about 15.4ppm. The fact that you lost so much chlorine overnight tells me that you are fighting something in the water. So, yes, you need to shock the pool up to 25 this evening after sundown. Test in the morning within 2 hours of sunup. If you have lost more than 1ppm of cl or if you have a CC reading greater than 0.5ppm, shock it up again. Mid-day tomorrow afternoon, test again if you can and add enough bleach to get back to 25. Do the same tomorrow evening and then try the overnight test again. When you can pass the overnight test, then you can let the cl drift down and keep it between 5-10ppm all the time.
Hope this helps!
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