There are basically three methods of pond filtration available

Filtration using Biological pond filters
Filtration using Mechanical pond filters
Chemical pond filtration

There are many types and styles of pond filters available including intake filters, submersible pond filters, in-line filters , external, and combination pond filters gravity fed filters up flow and down flow pond filters ,veggie filters.All utilize one or a combination of the methods mentioned above  The respective pros and cons of these various types of pond filters as well as their  general  care and maintenance are topics which will be covered in other posts, for the purpose of this post we are discussing biological pond filters and the role they play  in your pond within the nitrogen cycle . Because Biological pond filters come closest to mother natures own methods of keeping water clean and pond fish happy it is the most highly favored method of pond filtration chosen by most pond keepers.

The most crucial function of a biological  pond filter is to be a habitat for the beneficial bacteria that feed on  the organic matter in your garden pond . Any organic matter when it decays produces ammonia which in sufficient quantities is deadly to pond fish . When and where ever anything plant or animal dies and begins to rot, whether that be , a dead frog, fallen leaves, or fish poop a  natural process starts whose function is to decompose the organic matter in order to return the nutrients within it back to mother earth from where they came, this natural process is known as the nitrogen cycle.

In a  garden pond  the pond filters main function is to prevent ammonia levels reaching dangerous amounts sufficient to cause fish death and the prevention of algae. Naturally occurring bacteria known as Nitrosomonas in the pond filter will feed on the ammonia  and begin to convert it to nitrite. A whole new group of  bacteria known as Nitrobacter will take the nitrite and convert it again into nitrate. The Nitrates produced in the pond filter are released into the garden pond where they then become food for  aquatic plants  so completing the nutrient cycle. and demonstrating the importance of including water plants in any garden pond. When this natural cycle has become successfully established your garden pond is said to be  “balanced”.

Although the beneficial bacteria will given time, occur naturally inside the pond filter, they are often slow to colonize and are sensitive to oxygen levels and temperature change so many  pond keepers often employ purchased starter cultures of beneficial bacteria to speed along the natural process. Personally I have never used a purchased  bio-starter or bacteria starter to pre empt the natural colonization of  beneficial bacteria in my pond filter.  I have always been suspicious as to whether they really do contain what they claim they do . Nitrobacter and Nitrosomonas bacteria  are  sensitive to pH, salt,  temperature, and most other chemicals. they cannot survive any drying or heating or freezing process without killing the organism,they are so sensitive to levels of dissolved oxygen that if the pond filter or pond pump is turned off over night they die and a whole new colony must be started. So it seems illogical to believe these bottled and freeze dried bacterias really do what the manufacturers claim.

Affects of Temperature on Effectiveness of Pond Pump

The temperature for optimum growth of nitrifying bacteria is between 77-86° F (25-30° C).

Nitrifying bacteria will die at 32° F (0° C).

Nitrifying bacteria will die at 120° F (49° C)

Nitrobacter is even less tolerant of cold temperatures than Nitrosomonas. In cold areas garden ponds should be monitored ,for the accumulation of nitrites.

Affects of PH Level of Pond Water and Effectiveness of Pond Pumps

The optimum pH level for Nitrosomonas bacteria is between 7.8-8.0.

The optimum pH level for Nitrobacter bacteria  is between 7.7-8.2

At pH levels of 6.5 all growth of Nitrosomonas bacteria is drastically reduced  All nitrification of pond water completely ceases if the pH drops to 6.0 or less. Care must be taken to regularly check the  ammonia content of pond water both in the pond filter and the pond itself if the pH begins to drop close to 6.5. at this level of pH almost all of the ammonia present in the pond water will be  mildly toxic, to pond fish letting it fall beyond this level will mean fish death.

Levels of Dissolved Oxygen and Effectiveness of Pond Pumps

The best  nitrification rates  exist if dissolved oxygen  levels are  80% saturation or greater. Nitrification will not occur if  dissolved oxygen levels fall to less  than 2.0 mg/l (ppm) . Nitrobacter is more strongly affected by low dissolved oxygen than NITROSOMONAS.

So bearing all these parameters in mind I have never found it plausible that these freeze dried or bottled liquid forms of pond starters really work. I prefer to start off my pond filter with a culture of bacteria I know to be living .

You can beg borrow or steal  a portion of the green slime found in a the bio pond filter of a friend or neighbor who either themselves owns a garden pond or fish tank As Living in the Niagara region as we do it is not possible to keep the pond running year round so usually I make it part of my winter pond close down routine to capture two or three largish goldfish to house indoors in a tank to start off my bacteria colony for my pond filter the following year. Over the tank is suspended a grow light as in the top of the tank I usually bring in two or three water hyacinths to start off the pond in the spring rather than wait for local pond plant suppliers to receive their first shipment the quicker you can add floating plants to your pond surface the better you don’t have to do this in a fish tank any glass vase on a sunny windowsill will do the trick but you may find it necessary to add liquid plant food as there is no fish waste to be converted.

If this is not possible for you or in the case of a brand new pond first I de-chlorinate my pond with Sodium Thiosulfate (a  chemical that does actually work) I then add feeder fish or a number of other really cheap fish to the pond . Goldfish are not as hard to please as Koi as far as water quality is concerned and will usually be fine in a new pond so long as not too many are added in one go .A biological pond filter will automatically start itself up once fish are introduced to the garden  pond. Nitrosomonas and Nitrobacter are naturally occurring and will very quickly find their own way into your pond  filter. All they  need is oxygen from circulating pond  water  with a pond pump through the pond filter as well as excreted ammonia from the fish poop to thrive increase in population in the pond filter . It will take a while for the bio pond filter to reach “maturity” or in other words to contain a big enough colony of bacteria to consume all the amonia produced by the fish so some fish may be sacrificed to this process thats why I always start off with cheap fish not expensive large goldfish or koi.

By  keeping a watchful  eye on the nitrogen cycle within the pond filter system will give you a good idea of the overall quality or health of your pond water .It is a simple process to monitor all these different chemical levels  with either a basic and inexpensive  5 in 1  test strip or a more advanced  pond water test kit. both available online or at your local pond supply store they will easily help you determine if the optimum levels are being maintained  by your pond filters . If problems continually present themselves it is possible that a larger biological pond filter may be required. Other solutions depending on the particular problem may be cured by increasing oxygen by the addition of a pond aerator,or  fountain or even by the addition of more pond plants.
Ideal pond water readings should be

nitrites= zero,
ammonia = zero
a ph range of 6.8-7.8
nitrates=20ppm or lower
dissolved oxygen=9ppm or above.

Dissolved oxygen=9ppm or above is best for Koi gold fish and other pond fish will survive with less carp in the wild have been found to live in waters where only 4ppm is available but this is in much greater bodies of water than found in the average garden pond  Adequate dissolved oxygen is essential for good water quality. Oxygen is a is a required element for all forms of life. As dissolved oxygen levels in pond water drop below 5.0 ppm,all  aquatic life is put under stress. This is why a waterfall or fountain is is not just a eye catching piece of garden decor for a garden pond it is a a life sustaining essential part of your pond filter system .The lower the concentration of dissolved oxygen gets , the greater the stress on all aquatic life . Oxygen levels that remain below 1-2 ppm for a few hours can result in extensive death of pond fish.

If you can obtain pond water quality  readings as the figures  above on a consistant basis then you can be confident that your pond filter is doing a proper job and that your garden pond is a healthy and pleasant place for pond fish and pond plants to co exist in a mutually beneficial relationship.