They are far more important than you might have thought.
Stephen M. Meyer
Q. I am in the process of digging a natural pond. I was wondering, however, if I could avoid doing water changes. If I do not have to do water changes, do you think I could put high-quality koi in with goldfish? I plan to have koi, goldfish, plants and snails in the pond. It will be about 12 feet long by 8 feet wide, with a depth varying from 1 to 4 feet. Also, are there any koi clubs in my area?
A. Let's see — your planned pond holds about 1,300 gallons and has a surface area of roughly 70 square feet (assuming rounded corners, etc.). You plan on making this a natural pond, which means no more than one koi or several goldfish by any reasonable rule of thumb. And you want to know if you can avoid water changes. No, I don't think so.
In my recent foray into the aquatic ecology literature, I have gained a far greater appreciation for the impact of water changes — even relatively small (10 percent per week) water changes — on pond health. Water changes are mandatory unless it is your explicit intention to make a bog pond, in which case there can be no fish and only a very limited selection of plants.
Your ornamental "natural" pond will have no continuous inflow or outflow of water. From a biological and chemical standpoint, it will be stagnant. Recirculating water with a pump is no substitute for fresh water. The many biological, chemical and physical processes that take place in your pond will, over time, make it more acidic.
Eventually it will become too acidic to support the nitrifying bacteria needed to detoxify ammonia. The fish will start to show signs of stress-induced illness as the pH drops below 6.0. Meanwhile, concentrations of dissolved and suspended organic substances will increase and make the pond quite inhospitable to most plants and animals. As I said, it will become a bog pond.
Water changes counteract these natural processes by physically removing old water and replacing it with new water. While even this is a poor substitute for the much greater water turnover that occurs in productive natural ponds, it is literally better than nothing.
I do not understand your desire to avoid water changes. A simple 10-percent water change cannot take more than a half-hour per week, and you can use the old pond water to water the lawn or the garden because it will be very rich in nutrients and is an ideal organic fertilizer. It will also give you time to examine how your pond is functioning. Indeed, if your goal is to simulate a natural pond, then taking the time to sit and observe while water is being exchanged is part of the purpose in building it!
Pond water changes can be made effortlessly with a handy submersible utility pump and a garden hose. These pumps can be purchased at any hardware or building supply store for under $50. Every pond owner should always have one on hand for basic pond maintenance. Attach the garden hose to the utility pump and drain out 10 to 20 percent of the pond water. Then just fill the pond from the outdoor tap.
Of course, if your water is chlorinated then things are a bit more involved. You will need to use a chemical dechlorinator. If you change fairly small fractions (10 percent) of the pond water per cycle, there should be no problem in adding the dechlorinator to the pond as you refill with tap water.
If your area receives frequent substantial rainfall, you can let nature do some of the water changing for you. This will require some simple plumbing. You need an overflow that runs from the bottom of the pond (see the accompanying diagram).
As I have said before in this column, I do not recommend keeping koi and goldfish in the same pond. They can interbreed easily, often to the detriment of the goldfish, which are badly beaten up by the amorous advances of the much larger koi. I do not know of any koi clubs in Illinois, but if any of our readers do, please drop me a note.