Clear water does not necessarily mean clean water,
it may contain colorless impurities, such as ammonia and nitrite,
that are harmful and can kill pond fish.
Pond fish excrete urine and produce feces, and ammonia is excreted
through the gill membranes.
Also debris collects in ponds.
It is the job of the filtration system to remove waste which in
the wild would be diluted by the large volumes of water or washed
away by moving water.
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Mechanical Pond Filtration:
Most filter media have a mechanical function.
Settlement chambers allow gravity to drag the solid waste out of
the water by slowing the water flow. Such chambers usually come
first in a filter. A vortex unit provides greater settlement, the
water moves in a circular movement allowing solids to gather in
the center where they can be removed. In addition to baffle plates
which slow the incoming water, brushes or matting can be used to
strain the water.
Biological Pond Filtration:
This relies on specific bacteria to break down
toxic waste products to less harmful substances. There are two
stages in the breakdown of ammonia, each stage involving different
types of bacteria. The first stage is the breakdown of ammonia to
nitrite by nitrifying bacteria, most important of which is
Nitrosomonas. The second stage is the conversion of nitrite to
nitrate by Nitrobacter.
Chemical Pond Filtration:
Activated carbon removes ammonia and other organic
waste products by adsorption, this means that the waste substances
become linked to the surface of the carbon. When the surface is
'full up' it has to be replaced.
Zeolite removes ammonia and nitrite from the water. A good feature
of Zeolite is that it can be cleaned by soaking in salt water (6g
per litter) for 24 hours and then reused.
If a large biological filter is present chemical filtration should
not be needed, but it is good to use while the biological filter
is maturing or isn't big enough for the pond.
Sand filter. Some pond fish keepers use a sand filter as a final
stage to 'polish' the water. The water is passed under high
pressure through sand and comes out very clear, bacterial activity
also takes place in the sand filter. Sand filters are expensive
though, and you can't make one yourself because of the high
Biological filtration turns ammonia into nitrate
which is harmless to fish (unless at extremely high levels) but
the disadvantage of this is that algae love nitrate and you get an
algae bloom. There are two types of algae problems, green water
and blanket weed. Green water is caused by microscopic algae in
the water, it is not harmful to pond fish, actually it is
beneficial, the pond fish eat the algae and it enhances their
color, but you can't see them!
Also in summer the algae use oxygen and leave the fish gasping.
There are various ways to get rid of the algae : a vegetable
filter, plants will use the nitrate so it is not available for the
algae; an ultra violet (UV) filter kills the algae as it passes
through; algaecide chemicals can be used but the problem will just
recur; magnets placed on the filter pipe will disrupt algae cells
internally, killing them or preventing them reproducing. Blanket
weed is filaments algae and forms long green strands, it is not
really a problem, it uses up nitrate and stops green water
occurring, but it is unsightly. Vegetable filtration, algaecides
and magnets will all work on blanket weed, but UV filtration will
not as the algae has to pass through the filter to be killed and
blanket weed is attached to the pond walls.
Cleaning your filters:
Filters need to be cleaned occasionally to remove
sediment, take this into account when building one. Add a bottom
drain to each filter chamber so that sediment can be let out, it
makes cleaning much easier. Another thing that makes cleaning
easier is to put filter medium in net bags, not just pour it in
all at once, as it can then be more easily removed, one bag at a
One last important thing, never ever put tap water in a mature
filter, it will kill all the bacteria and you will have to let it
mature all over again.