In 18K gallons, each 1.5 gallons of 6% bleach will raise your FC by 5 ppm, so if you're starting at zero chlorine, then 8 gallons should get you over the 25 ppm mark with a little bit to spare.
Make sure you have some extra bleach on hand, though, because once you get it up to 25 ppm, you need to hold it there by testing and adding as many times a day as you can to keep it at or over the 25 ppm mark.
Janet
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