Choosing the correct pond filter media is essential to
maintaining a healthy Koi pond. Fish excrete urea, which becomes ammonia.
Ammonia is oxidized first into nitrite and then into nitrate by bacteria in the
water. Ammonia and nitrite are toxic to fish; nitrate is relatively harmless
and is taken up by aquatic plants as a nutrient.
Nitrifying bacteria exist on every surface in a pond but can only convert the
ammonia and nitrite that they physically contact into harmless nitrate. Biological
pond filters contain media that provides a substrate upon which colonies of
nitrifying bacteria can be concentrated. The entire volume of the pond is
circulated through the biological filter once every 60 to 90 minutes, forcing
all of the ammonia and nitrite in the water into contact with the bacteria.
These bacterial colonies live on surfaces. While they will colonize smooth
surfaces, such as marbles or pebbles, they cling better to irregular and ridged
surfaces. Filter media with higher surface areas support higher populations of
beneficial bacteria per volume of media. Small straw-like structures or blocks
of sponge, for example, would support more bacterial growth than small rods or
a solid block, because the straws and sponge provide both inner and outer
surfaces on which the bacteria can grow.
Gravel, rocks, engineered ceramics and zeolites are common pond filter media
and have been used for decades. Engineered plastic designs, however, are far
more popular now. Artificial media is lightweight, stable, and designed to have
extremely high surface areas. Bio balls are highly efficient filter media. They
are plastic spheres with hundreds of tiny rods extending from the surface of
the ball. These rods drastically increase the surface area of bio ball when
compared to a smooth ball of the same size. Polymer ribbons with embossed
ridges and grid patterns are also highly effective media in biological pond
filters. The ribbon is unspooled into the filter container and allows good
water flow, while the embossed ridges on the ribbon increase the surface area
for the bacteria to colonize by a factor of ten or more. Savio Springflo is a
good example of ribbon-like Koi pond filter media.
Some biological pond filters contain media in the form of fibrous mats or pads.
These mats vary in density, which impacts the flow characteristics of the pond
water being filtered. Course filters mats allow a high flow rate and filter out
large debris. Finer filters have higher densities and much more surface area
per cubic inch, but tend to impede flow and require greater head pressure to
move water through the filter. They also tend to become blinded by small
particulate matter more quickly than less dense media. Most filter designs pass
water first through a course, less dense media, then through successively more
dense media in order to prevent blinding of the fine mats.
Unfortunately, the nitrogen cycle is not perfect. Bacteria die and fall off the
media, and clumps of bacterial colonies are sometimes dislodged and swept away
in the water flow. Over time these materials accumulate and clog the filter. In
order to maintain good water flow over healthy bacterial colonies, the filter
media requires periodic cleaning. This is typically accomplished by
back-flushing the filter to an area outside of the pond.
Chemical cleaners or chlorinated water should NEVER be introduced into the
filter media; doing so will kill the bacteria and also add the toxic chemicals
or chlorine into the pond water. The goal of cleaning biological filter media
is removal of dead bacteria and sludge buildup. A healthy coating of nitrifying
bacteria must remain on the media or it will not be able to remove ammonia and
nitrite from the pond water.