Q. A friend and I are having an argument about water changes. I want to do one partial change of my pond water every month — something around 20 percent of the volume. I think this would have the greatest overall affect on maintaining good water quality (pH, nitrate, etc.) and would involve the least amount of work. My friend says that it would be better to do a 5-percent weekly water change. This would be a lot of extra work and I do not see why it would be any better. Who is right?
A. Well, I am not sure that I can offer a simple right-wrong answer. As the accompanying graph illustrates, both options result in comparable average ambient concentrations of pollutants. Your monthly approach actually offers a slightly lower monthly average, but at the price of fairly large monthly fluctuations in water characteristics. It is in this respect that averages can be deceiving.
Because both approaches produce roughly the same average result, I would opt for the one that results in the greatest stability in water conditions. You see, most pond fish can adjust within limits to a wide range of conditions if those conditions remain stable. What induces stress is frequent and large fluctuations in water quality parameters that prevent adaptation.
Your monthly changes result in swings of 20 percent every four weeks; your friend’s weekly changes result in swings of 5 percent in water parameters every week. From the fish’s perspective, the latter is preferred, especially if pH is one of the parameters that shifts with each water change.
A slightly larger weekly water change can have a disproportionately large impact on water quality. For example, an 8-percent weekly water change would produce a line that flattens out at around 1 part per million (ppm), compared to the average of 1.7 ppm for the 20-percent monthly or 5-percent weekly approaches. That produces a 40-percent drop in average pollutant concentrations.
Another thing favoring smaller weekly water changes is that they tend to be more manageable and they take less time overall. Moreover, while you are waiting around for the water to drain and then refill the pond, you can observe your fish closely. Frequent and routine observation is the best way to catch emerging fish health problems early.