There are hundreds of different commercial filters on the market today. Which one is best for you? That actually depends upon what type of pond you plan on filtering.
If you have a simple whiskey barrel filled with water, a small pump-and-fountain combination will work fine. If you add fish to the barrel, you should probably add some sort of biological filtration medium to your pump. Virtually anything will do. We use foam pads in our smaller ponds. They allow water to pass through easily while trapping larger waste products and encouraging bacteriological growth. When the flow from the pump slows to a trickle, it's a good bet the pads are dirty. Take them out and rinse them lightly with a garden hose. Don't wash out all of the beneficial bacteria, though, or you'll be defeating your own purpose.
If you have a larger pond with a number of goldfish or koi in it, you should definitely consider using a combination of filter types. A skimmer box that removes waste from the pond’s surface can be fitted with a foam pad and bundled straws for a combination of mechanical and biological filtration. You can purchase mesh bags of carbon and drop a couple of those into the skimmer box, too, once a month. In that way, you’ll have three types of filtration working for you—mechanical, chemical, and biological.
We have a skimmer box equipped with a heavy duty pump at one end of our main pond (the deep end) and a PondMaster filter in the shallow end. It contains a small pump, filter media, and a grate to keep the fish out. The entire unit sits submerged on the pond floor so that it's virtually invisible (well, to us, anyway--we're pretty sure the fish know it's there).
We also use a natural means of filtration known as a vegetative filter to help reduce the nitrate level of the water. It's a separate holding pond loaded with plants. As the water flows through the pond, it filters through the plants’ roots, and the plants take in nitrates while giving off oxygen.
Why go to all the trouble? Because we have koi.
Think of your koi as pigs with gills. No matter how lovable, they still generate an enormous amount of waste, which means they are living, breathing ammonia-making machines. Our bio-filters convert the ammonia to nitrites, which are toxic, and then to nitrates, which are less toxic. Our vegetative filters (plants) remove the nitrates. See the beauty of this setup?
Most koi pond owners don’t use vegetative filters because koi root up and swallow most plants nearly as quickly as you can introduce them. In that way, they’re a bit like infants. If they can reach it, they’re going to eat it. (That includes floating thermometers, by the way—so don’t use them!)
Enter the vegetative filter. When you run your pond water through a vegetative filter consisting of a heavily planted pond or even a stream bed, the plants' roots take in the nitrates and give off oxygen. It's a win-win situation for everyone--especially if you happen to be a koi!
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