Something to think about. The slime coat is the fish's main defenses against infection and disease. What's in tap water and why do we have to treat it? We treat it to protect the fish against these chemicals. Chlorine. Fluorine compounds. Trihalomethanes (THMs) Salts of: arsenic. radium. aluminium. copper. lead. mercury. cadmium. barium. Hormones. Nitrates. Every time you do a LARGE water change you put your fish at risk. The Microorganisms that live in all ponds and in your filter are what helps your fish thrive. Does your fish want tap water or fishy water? Something to think about.