It's more a matter of letting the chlorine finish, than it is FC levels alone. It's not FC that damages the cover; it's the volatile (can turn to a gaseous form) CC's that result was the high FC breaks down various 'goo' in the water. There are 2 reasons for keeping the cover off:
1. FC + UV (sunlight) are MUCH better at breaking stuff down, than FC alone.
2. The gases that form need to be released so (a) they are gone from your pool and (b) they don't accumulate under your cover and cause damage.


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