most of the responses to add ...
Published by Joseph Cotruvo, President at Joseph Cotruvo & Associates, Water , Environment and Public Health
most of the responses to add chemicals or UV are missing the point. I do not think that you are looking for more expenses and complications. A passive low cost system would be best. So, cover the tanks to prevent solar exposure. If you can't do that you can add floating plastic balls that cover the surface and block the sun. About 10 years ago Los Angeles had a problem in a chlorinated finished water reservoir because of solar catalysis for conversion of bromide to bromate. They covered the reservoir with floating black balls. Problem solved.
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