fish pond filters

Fish Pond Filters and Why They're Needed

In a modern water garden, housing goldfish or koi, fish pond filters are essential to the maintenance of a healthy pond ecosystem. Unlike a river or stream that constantly renews itself with clean water, a pond is a closed system, which means that anything that enters the pond is likely to stay until it is removed by some outside force. The easiest way to remove contaminants out of the pond is through the use of various filters and by performing regular partial water changes.

biological filter
A Pressurized Biological Filter

What Kind of Filters are Needed?

There are many natural forces at work in a pond, most of which are trying to their best to make your pond water dirty. By 'dirty' we don't just mean green water, or floating debris. Some of the most dangerous contaminants, such as ammonia, are invisible to the naked eye and can exist in even the clearest pond water.

There are two main types of filtration commonly used in koi ponds: mechanical filters such as pond skimmers, and biological filters.


A skimmer is a type of mechanical filter that uses some kind of net, screen or basket to trap floating debris. The most common floating material in a pond is usually organic; things like leaves, grass and sticks. It is very important to trap this material and remove it from the pond before it is allowed to sink to the bottom and decay. Decaying organic material produces ammonia, which is toxic to koi and other pond fish. To learn more about mechanical filtration, please see our page dedicated to pond skimmers.

Biological Filters

As we mentioned above, ammonia is a chemical that is toxic to koi and other pond fish. The highest source of ammonia in a pond is usually fish excrement. In high enough concentrations, koi will become sick and eventually die. To combat the problem of ammonia in ponds, biological filters were developed. A biological fish pond filter is a device that draws in water from the pond and exposes it to beneficial bacteria that are able to clean the water on a molecular level. The bacteria naturally convert toxic ammonia into nitrate, which is much more tolerable for koi. To learn more, please visit our extensive page on biological filtration.

Our Favorite Biological Filter for a Fish Pond

Waterfall Rocks

Aqua Ultraviolet Ultima II Pond Filters - The Ultima II series of filters from Aqua Ultraviolet are simply the best pressurized biological filters available on the market. While they cost a little more than some other filters on the market, the higher performance is well worth it. Features include integrated back-flush cycle for easy cleaning and high quality media that never needs to be replaced. The filter is pre-seeded with beneficial bacteria to start colonization quickly. Keep in mind that a pressurized filter can add between 5 to 10' of head presure to the pumping system depending on how dirty the filter is.

Ultima II Filter ModelFlow RatePond SizePrice 
Ultima II 1000 500 - 1200 gph 500 - 1000 gal $534.99 More Info
Ultima II 2000 1000 - 2500 gph 1000 - 2000 gal $705.99 More Info
Ultima II 4000 2000 - 4500 gph 2000 - 4000 gal $1,064.99 More Info
Ultima II 6000 3000 - 7500 gph 4000 - 6000 gal $1,424.99 More Info
Ultima II 10,000 5000 - 7500 gph 6000 - 10,000 gal $1,789.99 More Info
Ultima II 20,000 7000 - 7500 gph 10,000 - 20,000 gal $2,143.99 More Info
Ultima II 30,000 SMV 10,000 - 20,400 gph 20,000 - 30,000 gal $3,868.99 More Info
Ultima II 30,000 TMV 7000 - 7500 gph 20.000 - 30,000 gal $2,845.99 More Info
Ultima II 60,000 15,000 - 20,400 gph 30,000 - 60,000 gal $5,353.99 More Info

To learn more about pond filtration in general, please visit our page on pond filtration.

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