Maybe try a Chlorine Demand Bucket Test to help figure out what's going to help. I'd start tonight and do an overnight test of just plain pool water to see if it's something in the water or something else causing such high chlorine consumption. After tomorrow morning, start dosing the bucket to see what it will take to get chlorine to hold - if you can at all. Then figure out if the chlorine or the water will be cheaper. Also maybe also do a bucket test with half pool water and half fill water to see if that's going to help.