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What causes the results of a total and free chlorine check in a swimming pool to turn a brown to black color?



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What type of test reagent are you using. All reagents have a shelf life and should be at least replaced at the beginning of every season. If you just purchased it there is a chance it may have been merchandise left over from last year. Taylor testing products are the most reliable I have found. Check out their website for more tips on testing your water.