My book on Koi care written by Doc Johnson says that there is not safe
level for Chlorine exposure.

Chloramine is worse, and can be added by your local water district,
at any time, and without notice. It can also be spurious. They may
dose with it for a while, and then stop. The chemistry from local
water districts isn't constant, because they have to treat water in
response to contaminants. Any automated water system for ornamental
fish should be structured to assume the worst.

C//