koi pond

koi pond

Q. We have just installed a new 1800 gallon pond. We already have three koi and plan to stock it with about another dozen. The pond filtration water treatment system is designed around 80 cubic feet of crushed stone to provide biological filtration. My question concerns pre-treating tap water. We have chlorine in our water, with levels often reaching 1 part per million (ppm; equal to 1 milligram/liter). We treated the 1800 gallons when the pond was filled, but I am wondering if we will have to do the same when we make water changes. In particular, we plan on weekly water changes of roughly 200 gallons. This is about 11 percent of the pond volume. Could we put untreated water directly into the pond, or perhaps through the filter and then into the pond? Buce and Anie Delon, Utah.

A. Fish are very sensitive to chlorine. Concentrations as low as 0.001 ppm (one 1/1000 th of your water’s concentration) will often cause fish to try to escape from the offending water source. In other words, they will attempt to avoid even very low levels of chlorine like the plague ! Toxicity studies on freshwater fish suggest that the highest allowable concentration of chlorine that does not appear to show demonstrable harmful effects is around 0.01 ppm. However, I emphasize again that the fish themselves seek relief from concentrations one-tenth this amount.

Currently, aquaculture standards state that long-term exposure to chlorine should not exceed 0.003 ppm. Other tests suggest that the minimum allowable short-term (30 minute) contact with chlorine should not exceed concentrations of 0.05 ppm. Extended contact with such concentrations has been shown at minimum to affect the fertility of freshwater fish. If you plan to raise some prize show koi and possibly breed them, this should certainly concern you. And, even if breeding is not of interest, the mere fact that such effects exist should make you nervous.

They suggest that other, unobserved, health effects may also be present. Now, your proposed water changing scheme would have you dilute 200 gallons of 1 ppm chlorine with 1600 gallons of essentially 0 ppm chlorine. This will result in a temporary pond concentration of about 0.11 ppm chlorine. Interaction with organic materials in the pond water and with the air will see the chlorine dissipate over the course of a day. I would argue that this temporary exposure is far too high to be considered inconsequential. Although the fish may not show immediate physical harm, it is likely that over time you will see cumulative health effects. Unfortunately, by the time they become apparent, it will be too late to reverse them. You certainly do not want to route this water through the biological filter.

Chlorine is used to treat drinking water to control bacteria and other human pathogens. Because the heart of the biological filter is the population of nitrifying bacteria, running chlorinated water through your filter will seriously impair its operation. You have two options, in my view. First, if you have simple chlorine in your water, rather than the more complex chloramine, you might try fine spraying the new water over the pond when making water changes. This greatly increases the dissipation of chlorine into the air. You might try this approach once, wait 10 minutes and then measure chlorine levels in your pond. If they are immeasurable, you are probably safe (I say probably because some hobbyist test kits have fairly high minimum thresholds). Alternatively, you could rig up a holding tank where you could treat the water prior to adding it to the pond Three large trash pails will hold about 90 gallons, for example. Chemical pre-treatment may be expensive and time consuming, but it does ensure that no chlorine reaches the fish.