Due to its very reactive nature, chlorine dioxide is rapidly (in a few hours) reduced to chlorite, which is persistent also as a biocide but 16 times less toxic to fish, according to MATC. Therefore, it is much more likely that fish will be exposed to chlorite than to chlorine dioxide in natural waters. Presently accepted, the Maximum-Permitted-Concentration of total residual chlorine (TRC) in waste-water discharging into receiving waters is 0.6 mg/l. If this requirement will not be exceeded, it is unlikely that fish would be exposed to lethal or even to sublethal concentrations of chlorine dioxide or chlorite. Furthermore, chlorine dioxide does not generate toxic nitrogenous (chloramines) or carcinogenic organic residuals (trihalomethanes). All these properties make chlorine dioxide a more promising biocide than chlorine.

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Abstract. The effect of copper ions in water on the growth of rainbow trout (Oncorhynchus mykiss) larvae and abundance of bacteria in the intestinal tract was studied. Exposure to copper reduced the growth of fish larvae and the abundance of bacteria in the intestinal tract. Populations of aerobic total heterotrophic bacteria present in the intestinal tract of rainbow trout larvae, estimated using the dilution plate technique, decreased approximately from log 5.30 CFU/g in control larvae to log 2.39 CFU/g after the exposure to 0.03 mg/l of copper. Proteolytic bacteria proved to be the most sensitive to copper. The abundance of these bacteria in the intestinal tract of larvae decreased significantly after the exposure to the lowest tested copper concentration. In these experiments the cultivable bacterial densities increased after the exposure to 0.25 mg/l of copper, possibly as a result of the release of organic carbon from the poisoned copper-sensitive organisms.