Tap wafer is chlorinated because chlorine is an oxidizing agent fortifier by nature .chlorine is able to oxidize most disease germs to prevent diseases from, spreading through water ,yet, some chlorine will remain in the water which is called ¡°residual chlorine.¡± The residual chlorine will poison the mucus membranes and gills of fishes;so, tap water has to tested after a water change.
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