I built a very simple automatic water change system in my front pond. It has two components, a reservoir and an overflow pipe.
First, we have the overflow pipe. I made a notch in the wall of the pond big enough for a 2 inch pvc pipe. If I just put a pipe in this notch and wrap the liner snugly around it, I have a simple overflow pipe that will direct excess water out of the pond when it rains. But I wanted the excess water to come from the bottom of the pond. So I put an elbow on the horizontal pipe, and cut another piece of pipe long enough to reach almost to the bottom. Sticking this on the elbow, gives me a drainage pipe that will drain water from the bottom of the pond when it rains, actually changing the water. It could also drain fish from the bottom of the pond, so I capped the end of the pipe and drilled a large number of 3/8" holes in the lower part of the pipe. Rotated so the whole thing is out of the water, it looks like this:
While it is partially covered by a hose, you can see another elbow on the outside end of the pipe. That's mainly there to hold the system in place.
While the overflow system worked as shown here, I later put the pipe through the liner using a bulkhead fitting. It works and looks better that way.
Now the reservoir. I used the tank of my old water softener for my reservoir. It's a nice cylindrical 30 gallon tank. I had to cut off the top, because there were a couple of little holes there, Then I calculated the volume, and made marks on the inside to indicate how much water was in the tank.
I put a hose bib (faucet) near the bottom of the tank. This required drilling a hole just big enough to slip it through. Then I used rubbery gaskets and locking washers on the inside and outside to hold it in place, then slathered it with lots of silicone caulk to seal it. Once determined that it didn't leak, I spray painted the barrel black.
You can ask for help at the hardware store with what you need to install the faucet. I suggest you tell them that you are making a rain barrel. They know what that is and the installation procedure is the same.
Initially, I planned to simply turn the faucet on to a drip. But when I tried this, after a few hours the drip stopped. So I got an adapter for the faucet that allowed me to attach 1/4 inch irrigation tubing http://www.homedepot...catalogId=10053 to the faucet. Then I got an adjustable dripper for the end of the tubing. http://www.homedepot...catalogId=10053
This allowed me to put the reservoir anywhere I wanted and have the clean water dripping into the pond anywhere I wanted.
I chose to have the dripper at the same end of the pond as where my flowerpot filter returned water to the pond.
The best place for my reservoir was at the opposite end, right by the overflow pipe.
I searched long and hard for a cover for my reservoir. I wanted a container that would fit over the top. I would put potted plants in the container to make the reservoir look like a big flowerpot. I found this feeder pan at Tractor Supply Company.
Here's the whole pond.
At in the foreground, to the right of the birdbath, you can see the overflow pipe coming out of the pond. To the right of that is the reservoir, with a poinsettia in the top for seasonal color. In the back, my flowerpot filter sits on the left, and the dripper is at the right back corner, not visible in this picture.
The system works beautifully. When I initially filled the reservoir, I added dechlor, but as long as I fill it up every few days, there's no need to add more since it takes a few days for the water to drip out so the water is aged.
I set the dripper to a fast drip, which adds about 5-6 gallons a day (35 - 42 gallons a week) to my 250 gallon pond. Of course, it isn't really that big a water change, since the new water mixes with the old, but a rainfall can produce a big water change. Furthermore, every week or two, I flush the filter. This involves draining the water from the filter, allowing it the refill, then draining it again. This removes 20 gallons of water which will be replaced by the clean water drip. The water is sparkling clear, odorless, and has perfect parameters. The goldfish are happy because any changes in the water are very gradual, and goldfish, like old men, like things to stay the same.
One warning. If you have acid rain, and particularly if you also have soft water, the passive water change from a heavy storm could result in a dangerous pH drop. If I had that situation, before a storm I would rotate the overflow pipe to the position shown in the first picture , so that the overflow water would come from the top of the pond, and thus be mostly rainwater.