I'm not sure what kind of filter you have, but mine is a Hayward sand filter. My pool volume is a little less than yours too (18,000). I can spend 10-15 minutes vacuuming to waste and only drop the level down about 2-3 inches. That's probably 300 to 800 hundred gallons, but a small percentage of the total volume.

I'd check the Cl at least twice a day (morning/night) and add the bleach you need to maintain the 12 ppm. Once you get things under control, you can test less often.