If your chlorine loss is during the day with sunlight on the pool, then yes this is because your Cyanuric Acid (CYA) level is 0. Near a pH of 7.5 in a pool of average pool depth of 4.5 feet with the sun directly overhead, the FC will drop in half in roughly one hour.
If you have the CYA in a sock, then it may dissolve in a couple of days in cold water -- in warmer water it usually takes less than a day in a return flow (i.e. if you run the pump 24/7). You can squeeze the sock in a day to move things along.
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