It may be that some of your CYA biodegraded. That is pretty common and we see it a lot of times in the spring when people open their pools. If it biodegrades, it can leave ammonia as a byproduct which causes a huge chlorine demand to get rid of it. I have a feeling that is what is going on in your pool. Or, you may have some other type of organic in the water that you cannot see that is causing the demand. Sometimes a pool can be on the verge of an algae bloom without having any visible signs and you go through a lot of chlorine. I highly doubt that having hard water would have any effect on chlorine use.
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