Recommendations for pond water changes are usually 10-20% a week, although a few go as high as 30%. This is a lot of water in koi ponds that are typically measured in tens of thousands of gallons. You run a hose for a long time to add hundreds of gallons of fresh water. Many pond keepers have at some time become involved in something else while the hose was running and forgot to turn it off, sometimes losing an entire pond of fish to chlorine. Even without such a horrible mistake, filling a pond can be risky since koi are drawn to running water and often gulp the incoming water, chlorine and all. This is one of several reasons why to many ponders, water changes are feared.
Many people have gone to a "trickle in-trickle out" water change system, in which a hose is left running at just a little trickle all the time. The amount of water entering is small enough that chlorine is diluted, and all large ponds have an overflow in the system somewhere where the water can trickle out. This produces a "steady state" situation in which the water chemistry remains constant, just as it would in a lake.
If one tries to do this in a typical goldfish pond, with volumes in the hundreds of gallons, one finds that you can't turn a faucet low enough to produce the tiny trickle you need without the flow just stopping after a while. (I tried. So have others.) While most liner ponds have a low spot somewhere for overflow, a container pond just has water spilling over the top.
The solution is to put the fresh water into a large container equipped to drip water into the pond and to create an overflow pipe in the wall of the container or through the liner of the pond to let out the excess water. I first did this in my front pond and described it here. My reservoir was the thirty gallon tank of my old water softener. I had not yet discovered uniseals, so putting the hose bibb (faucet) into the side was a difficult and expensive procedure, involving a lot of sealant. It leaked on the first try, but I succeeded on the second. Then I filled the reservoir with water and turned on the faucet to a fast drip/slow trickle. It worked beautifully for a couple of hours, then stopped. The water was flowing through such a tiny crack that even tiny bits of debris can clog it, and there is always debris outdoors. I have some drip irrigation for my garden so I went to the drip irrigation section at Home Depot to see what might work. I found an adapter to connect a garden hose/hose bibb to 1/4 inch irrigation tubing. Then I found an adjustable dripper that fit into the 1/4 inch tubing. It has a screw-on end that can be tightened to cut off the flow completely, or loosened to a slow drip, fast drip, or even a continuous flow. Perfect! (Links to these products can be found in the link above.)
To go through a liner, you need a bulkhead, which is something that is not easy to find in stores. Pond stores have them, but the one here charges 3 times what I paid on line. The bulkhead sandwiches the liner between two gaskets and has threads on each side to attach pipe.
Most overflow pipes simply skim water off the top of the pond. I run the pipe to the bottom of the pond to overflow the dirtiest water. When it rains, the rain falls on the top of the pond the pipe picks up the old water at the bottom of the pond and overflows that. As a result, every rainfall gives me a free water change. Don't use this system if you have soft water and acid rain, since that could cause a pH crash. Just let the pipe collect water from the top.
I adapted this system to a 50 gallon stock tank pond as described in post #10 here. This was much easier, since I used uniseals to go through the wall of the reservoir bucket for the dripper, and through the wall of the stock tank for the overflow pipe. Each are ten minute jobs. It took two days to put the faucet in the reservoir for the front pond, since I had to let the sealant cure. No sealant with uniseals.
Overflows from container ponds are all very much alike. However I have tried several designs for dripping water from reservoirs into the pond. I will describe and picture these in the next post.