FAQ - Pond - Filters

Filtration is the removal of unwanted items from a particular medium. In the case of ponds, the unwanted items are particles of pond debris, old fish food, dust, and other miscellaneous items that might be in the water, the medium. This is called “mechanical filtration” for the obvious reason that it is the mechanical removal of particles from the water. But in a pond there is a chemical process that needs to occur, as well, and perhaps more importantly. It is the process by which nitrogen, which is released as ammonia when protein decays, is converted from ammoniacal nitrogen (NH3, NH4), which is poisonous, to oxides of nitrogen (NO2, NO3), which are plant food.

-Mechanical filtration -
As stated above, this is merely the removal of particles from the medium, water. This can range from a grate that catches tennis shoes and basketballs from city storm drains to ultra-fine filters that can remove bacteria from water. A good pond filter falls somewhere in between these two extremes. Traditionally, people used pool filters, which depend upon fine silica sand as a medium. This is great in a pool, but in a pond, it plugs up very quickly and becomes more of a hinderance than an asset to the pond. The next step in the evolution of pond filters was the modified pool sand filter, in which the laterals are punched full of holes, usually with a screwdriver, to allow more flow, and the sand was replaced with lava rock. This is a method that is still widely used today and which is very effective. The lava catches enough material to be called a reasonably good mechanical filter, and it supports a large bacteria population. Today, there is a plethora of new filters available for ponds of all sizes and fish loads. Some of them use foam mats for the medium, which works well but plugs up pretty quickly, while others use various variations on the “bio-balls” theme, wherein a plastic object is produced with a large surface area and lots of open space to allow a free flow of water. All of these products are good and will provide water clarity and a healthy pond if maintained properly.

- Biological filtration -
It is less important to remove particles from water than it is to process nitrogen, so if there is to be a compromise between mechanical and biological, err on the side of biological. In other words, it is better to allow particles below a certain size to escape back into the pond and to convert a great deal of ammonia to nitrate, than it is to catch every little thing down to a micron or less and in the process slow the water down to the point where the bacteria have a hard time living. The bacteria that do this work for us are among a class of bacteria that we have all heard of before, they are the so-called, “nitrogen fixing” bacteria. This means that they take nitrogen that is unavailable to plants in its ammoniacal form, and make it available to plants in an oxidized form. These are the same bacteria that live among the roots of leguminous plants. Without these beneficial bacteria, life as we know it would cease. So be nice to your bacteria. What they want is a large surface area, chemically inert medium and a ready supply of fresh water. They depend upon dissolved oxygen in the water to live and to do their job. As soon as the water flow is stopped, the oxygen in the filter becomes finite, and eventually gets used up. The ultimate result is that the bacteria die, and you have to start over. Some builders use both mechanical and biological filtration, and some of the new equipment available does this. In the first instance, many people are tempted to install a canister filter ahead of the lava rock filter, thinking that they will extend the life of the lava rock filter by catching all of the particles that would eventually clog it up. They are correct, of course, but they wind up being slaves to the cartridge filters in an effort not to destroy their pumps, which have to work too hard to push water through the cartridges. It is better to merely use the lava filter and to maintain it well. Putting the canister after the lava filter is a good idea, but the same problem arises, just less often. Several types of bacteria are currently available to you, each with its own specialty. It is important to start out with a good supply of beneficial bacteria so that you minimize the green water stage of pond establishment.

-U.V. Clarification -
The theory is that if you pass the water past the UV bulb, uni-cellular organisms will be killed, and of course, this is true. Many people depend upon it to keep uni-cellular algae at bay. My thinking has always been that if the water is green, or wants to be, there is a problem that can be addressed more thoroughly than with a UV light. UV lights will not control string algae, blue-green algae, or large fish parasites. One mistake made by many people is that they leave their UV lights on when they are trying to kill string algae with live bacteria. UV lights are best used in those cases where there are a lot of fish, because it will kill many small fish pathogens, such as “Ich” and fungi. For this reason, it is especially important to have them on in the late winter and early spring months when the pathogens are active before the fish are fully active.

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FAQ - Pond - Filters

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