Filters play an integral role in fish keeping. They are also valuable—although not absolutely essential—for water features without fish. They help to keep pond water clean and clear, they can remove green algae from the water, and they extend the life of most pumps.

But with all the filters on the market today, what type should you use? That’s a good question and one that doesn’t always have a simple answer.

As you might imagine, different types of filters do the same job in different ways. They filter, or strain, debris from the water. Some filters are good at removing large pieces of debris; others are good at removing smaller particles. Although different filter types employ different filtration principles, all end up processing waste.

Of the different types of filters on the market today—from mechanical, vortex, ultraviolet, and sand filters to biological, vacuum, skimmer, and trickle tower filters—the most popular is the mechanical filter. It strains debris from the water, usually by means of some sort of membrane (called filter media) through which the water passes and in which the debris gets trapped. Most of the waste you’re going to be removing from your pond will come from your fish. I know this for a fact, because we have had fish ponds and we have had non-fish ponds, and believe me, fish ponds are messier!

Garbage in, Garbage out
Think of a pond as a giant trash can. What you put into it stays in it. A filter, contrary to popular belief, doesn’t remove waste. It simply collects it. You have to remove the waste, or it will remain there inside the filter, contaminating the water in one form or another, forever.

That’s worth repeating: What goes into your pond stays in your pond until you remove it.

Dry fish food? It’s there. Oh, it will have changed from fish food to fish feces, but the waste matter is still there. Chemical additives? They’re there. Salt? Medication? Fallen leaves? Still there. The only time that you actually clean your water of all those waste products is when you back-flush, clean out, or replace your filter. That’s one of the reasons you need to be diligent about maintaining filters, as well as about making frequent partial water changes. Water changes reduce waste matter, including potentially toxic chemicals, through dilution. A good rule of thumb is to provide a 10-percent water change each week for fish ponds and as often as necessary for non-fish ponds.

What about a naturally occurring pond? Who changes the filter there?

A natural pond contains far fewer fish than a man-made fish pond. In a natural pond, the water is constantly being replaced—usually more than 10 percent a week! Every time it rains or a stream overflows or somebody leaves his sprinklers on too long, the water makes its way downhill and eventually ends up in some creek, stream, or pond.
The number of fish and other creatures a natural pond can support was established eons ago by nature. That’s not the case with man-made ponds. Of course, we could build ponds and stock them in the same ratio as nature intended...say, one fish for every 500 gallons of water. But that wouldn’t be much fun.

That’s where the filter comes into play.

Filtration, or the act of removing waste from water, is accomplished in the pond primarily via one of four means: mechanical, chemical, biological, and ultraviolet.

Mechanical Filtration
“Mechanical filtration” is a means whereby waste is physically strained from the water. Just as if you were to take a net to clear the surface of your pond of fallen leaves, a mechanical filter sifts the waste from the water. A good example of a mechanical filter is a “skimmer box.”

Designed to “skim” the surface of your pond, a skimmer has an intake manifold at approximately the same height and a little below the pond’s surface. A pump inside the box draws the water through a filtration pad inside the box. As the pump moves water from the box out through a hose, more water is drawn into the box through the intake, where it passes through the filtration pad. This ongoing cycle is very efficient at removing most debris from a water feature. To clean the pad, simply remove it and flush with a garden hose.

Another type of mechanical filter is a “settlement tank." It’s usually a flat tank situated next to the pond. It works by pumping water from the bottom of a pond into the tank, where the collected debris settles to the bottom. The debris remains there until it’s removed.

A more elaborate type of settlement tank is a "vortex filter." It’s usually cone-shaped, ranging from 2 – 6 feet in height or higher. As water passes through the filter, it swirls around the side of the tank in a circular motion (thus, “vortex”). The waste settles on the bottom where it remains until it’s removed.

Check out D. J. Herda's two latest gardening books, Zen & the Art of Pond Building and From Container to Kitchen: Growing Fruits and Vegetables in Pots, both available from Amazon.com.

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