At this point ... organics (algae). All that time with little to no FC caused it I'm sure. Even if the pool seems clear or you see the hint of cloudiness across your pool, that's organics. FC only drops like that from sunlight and/or organics. So now is the time to "maintain" a SLAM FC based on your CYA. So according to the SLAM page (link below), first ensure your pH is lowered to about 7.2, then increase FC to "28" based on the CYA of 70. Keep it that way and follow all the other instructions on the SLAM page. Read that page at least nice daily to remind yourself of the SLAM details, and keep maintaining the FC of 28 as constant as you can (test as frequently as needed to confirm) until you pass all 3 SLAM criteria. Good luck!What could be causing me to lose FC so rapidly?