When to Use External and Submersible Pond Filters
By Barry Graham

Inside or outside the pond? That is the eternal question, and it applies to both pond pumps and to filters.

Generally, smaller ponds can use in-pond pumps and filters, but very large ponds require much larger equipment to be able to cope with the volume of water in the pond.

There are, of course different types of filters, primarily:

· mechanical filters that literally filter dirt and algae, and

· biological filters that break down waste in the pond water.

Both types are manufactured for use either in or out of the water, and you will find that the different types look very different to one another. For instance the size and shape of filters varies incredibly, ranging from what look like relatively small boxes to rather enormous cylinders. And both may be used in conjunction with an ultra-violet (UV) clarifier or stabilizer that will destroy algae cells.

All filters, mechanical, biological, in-pond (or submersible) and external, need a pump to move the water through the filter mechanism. If the filter is outside the pond, there will be additional pipe-work which can complicate installation. Think swimming pools for an example of a typical mechanical filter housed out of the water. These are usually high-rate sand filters that need a very powerful pump and quite extensive plumbing – and they are not normally used for ponds.

When it comes to garden ponds where mechanical filters are used, these are more commonly the type that stays in the water. The simplest type of submersible filter has some sort of screen – open-cell foam for example – that fits on the inlet side of the pump. As the water is drawn into the filter, the screen prevents algae, dirt and debris from entering the workings of the pump. These screens can get clogged quite quickly and need to be cleaned regularly.

More complicated external pond filters filters have layers of foam or matting and sometimes other filter media like coarse sand or gravel. These create some biological action, but because the pump is not usually left running all the time, when it is off, the biological action is halted, and it takes time to set up again.

External biological filters usually look more like cylindrical tanks. Both these and the smaller box-like types that are submerged inside the pond, have layers of filtration media that the water filters through. There is also live bacteria in the filter bed that converts waste products (some of which are toxic) into harmless material, including nitrates that feed plant life in the pond. Because it takes weeks for bacteria colonies to grow large enough to become effective filters, the pump needs to run constantly for a biological filter to work properly.

Ultra-violet (UV) clarifiers are housed in a waterproof casing so that they can be submerged in the pond. Pipe-work connects the unit, which contains a UV bulb that kills algae, viruses, certain fish parasites, and some harmful bacteria, to both the filter and the pump – either in or out of the pond. These units work best with an external biological filter. They can’t be used in place of a filter.

Barry Graham writes on a variety of topics pertaining to pond and water garden maintenance.

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