Tap water is formulated for human consumption and generally contains safe amounts of disinfectant chemicals to kill bacteria that could potentially make us sick. This is all well and good for the people in your household, but those same properties can spell disaster for your fish, especially sensitive (and valuable) koi. Chlorine and its derivative, chloramine, is highly toxic to fish and can cause painful or deadly burns in gills and even internal organs when in high enough concentration. Fortunately, there are a number of ways to mitigate these risks, depending on the size of your pond, the inhabitants, and the volume and makeup of the tap water you plan to add.
Many (but not all) municipalities now use chloramine rather than chlorine as a disinfectant because it lasts longer and does not evaporate. This means that the traditional practice of allowing water to stand for 24-48 hours to allow chlorine to evaporate naturally may be completely ineffective. It’s helpful to contact your local water company to see what chemicals they use. If chloramine is present, you’ll need to use a more intensive approach before you top off your pond with tap water. For example, a simple product like a dechlorinator or water conditioner can be practical for treating a relatively small volume of water in a small or medium size pond. Be sure to take a moment and make sure that your choice eliminates both chlorine and chloramine. For medium or larger installations that host fish, an activated carbon filter is an excellent choice that can improve overall water quality, eliminate both chlorine and chloramine, as well as filter out environmental contaminants like pesticides and tannins. To really seal the deal, activated charcoal can simply be added to your pond’s existing filter box and will help to support and maintain the growth of beneficial bacteria with relatively little effort. A setup like this will make topping off with tap water in relatively small amounts almost effortless.
