Pond Filters explained
Author: Barry Goodwin
Pond Filters are at the heart of a good Koi pond system, and the reason for having a filter,
and how it works, and how it will create a more livable environment for our Koi are not easily
understood by beginners. Indeed some older hands remain baffled about the more subtle aspects
of filtration. It is simpler if you have moved from tropical fishkeeping to Koi keeping, as
then you will, hopefully, understand the nitrogen cycle. That is one big step, but crossing
the hurdle of tropical requirements to those of Koi is sometimes more of a quantum leap than
some of us like to readily admit.
A leading question
Why do I need a filter and will it keep my water clear? Is a question often asked at the beginning,
so that is more than likely the best place to make a start.
Filtration is employed to remove ammonia from the pond water and not specifically to keep it clear.
You can have an efficient filter that does not give you clear water, although it must be said that
the two do normally go hand-in-hand.
Koi excrete ammonia directly from their gills as the disposal of part of their nitrogenous waste.
Ammonia also comes from the breakdown of plant waste, of solid and liquid waste from the Koi, and
the breakdown of all manners of other organic debris in the pond. In the enclosed pond environment,
ammonia will accumulate and build up to toxic levels for the Koi.
To prevent damage to, or at worst the death of Koi, this ammonia must be removed from the water.
This is carried out biologically through the provision of a suitable filter.
The Nitrogen Cycle
In a biological filter, ammonia is broken down by naturally occurring bacteria, firstly to nitrite,
which is also poisonous to Koi, and ultimately to nitrate. In the quantities we can expect it,
nitrate is not directly poisonous to Koi, although it can be a cause of blanket weed. Nitrate can
be removed by other means such as a vegetable filter, or a denitrifying filter that employs
different bacteria to convert it to gaseous nitrogen and release it to atmosphere. This biological
process is referred to as "The Nitrogen Cycle".
In reality, the Nitrogen Cycle works throughout the pond, not in the filter alone. All surfaces
will support the necessary nitrifying bacteria. A filter, with the provision of large amounts of
medium for bacteria to grow upon, ensures that enough biological action takes place to cope with
the ammonia produced by the Koi. The 'biomass' (the bacterial colony) will establish itself to
the load imposed upon it therefore it is keyed to the stocking level of fish living in the system.
Filter Size
DIY filter systems
Having accepted the basic idea of a filter, the next question usually is "how big should it be?"
and this depends on whether you intend to build your own filter system, or opt for a commercial
unit. For the building of a DIY filter, there are fortunately "rules of thumb" that have been
established for some time to guide us through.
If we consider that a suitable Koi pond should be between 4 and 6 feet in depth, we can see that
an "average" depth can be taken as 5 feet. To then create a pond of a size that will provide an
acceptable amenity in the garden, we need something not smaller than about six feet by twelve feet
in size. This will give us a pond of 360 cubic feet in volume, and the water capacity can be
worked out from this by multiplying the volume in cubic feet by a factor of 6.25. Therefore, in
the case of our hypothetical pond this will indicate a capacity of 2250 gallons (multiplied by
4.55 to give 10237 litres)- ideal for a smaller Koi pond.
We can then apply our "rule of thumb" for filter size, and say that for a pond of this depth,
using an "average" media such as gravel, that the filter plan surface area should be between 1/3
and 1/2 of the pond plan surface area. This will indicate a size of between 24 square feet (4' by 6'),
and 36 square feet (6' by 6'). It should contain submerged medium, we did mention gravel, in a
layer about 9" - 12" thick. If you wish to utilise a more modern, and perhaps more efficient medium,
then there are a number of lightweight plastic mouldings available that can be bagged in nylon mesh
bags for easy removal and cleaning. These are sold under various trade names such as "Flocor",
or "Bacballs" etc.
A more 'solid' medium that has a very large area is known as "Alfa Grog" and this finds favour
with many. It is far more efficient than the equivalent amount of gravel and therefore you should
be able to use less. Take your dealer's advice. There should be provision made during the filter's
design for flushing away solid waste matter to prevent its accumulation. If waste matter is allowed
to accumulate, it can provide a haven for pathogens (disease causing organisms) that will strike
when you least expect them. It can cause eventual filter blockage.
Commercial filter systems
There are many systems on the market that have been designed specifically to cater for the Koi pond.
These may be classified by the number of gallons they will support. Always check with your Koi dealer,
as a more accurate assessment can be made depending on the size and number of Koi in your pond.
If you intend to stock more Koi than the recommendations
for the filter of your choice, you may need to consider a bigger filter, or the doubling up of systems.
When shopping, always take your pond details with you, and then your Koi dealer will be able to make
sound recommendations.
Flow rates and quantities of medium may vary from those recommended in the 'DIY' advice given
above, but modern and efficient filter systems make better use of medium, and all other requirements
are adjusted to suit.
Water Circulation
DIY Filtration
The system's water must be circulated through the filter at a specified rate, and here again
a "rule of thumb" applies. We can say that for a system of up to about 55,000 litres, the total
amount of water must be turned over through the filter once every two hours. Over 55,000 litres
capacity, this rate is not so important and can be slowed somewhat. Therefore, if your pond has
a capacity of 9,100 litres, and you have 1,365 litres of water in your filter, then your system
capacity will be 10,464 litres of water. The flow should therefore be in the region of 5232
litres per hour. However, if you have a lot of tortuous pipe runs, and a lot of accessories
such as UV's and Venturis, then these will slow the rated flow of your pump considerably. This
must then be taken into consideration.
In a hypothetical pond of 11,375 litres, it can be that to maintain a true flow of 5118 litres
per hour, we will need a pump of up to 6825 litres per hour to give us the desired flow. When
purchasing a pump, then always describe the system, its pipe runs, and accessories to your dealer
and then he will be able to advise accordingly.
There must also be a certain "residence time" for the water in the filter medium. This should
be about 20 minutes, and it can be seen that for a flow of 5686 litres per hour, to obtain the
correct dwell time, the filter needs to contain about 1911 litres of water as well as the medium.
The foregoing suggestions are for DIY filtration systems, and if you wish to opt for a ready-made
system, there are a lot of these to choose from. Most such systems operate using slightly
different criteria, and once again you should take your dealer's advice.
Filtration success all boils down to learning as much about the technicalities of what you are
doing as possible, paying attention to detail, acquiring knowledge, and applying it in the
best way you can.
Commercial filtration
Broadly speaking, it is still correct for a commercial system to have the same turnover rate
as a DIY system, but mostly this will make a nonsense out of the recommended 'residence time'
for water in the medium. The answer to this one is that because such a filter makes more
efficient use of it's filter medium, it can achieve the same ends with less residence time.
Methods of driving filters
Gravity fed pond filters
Looking at a typical filter system it is generally accepted that for the best efficiency
the water must be gravity fed around the system to enable settlement, where solid matter
is separated from the water, to take place. In a gravity fed system the filter is usually
mounted with its water level at the same level as that of the pond. It will have large
diameter inflows, and the pump/s will be situated at the end of the system where they take
water from the filter, and return it to the pond. The water in the filter is replenished
from the pond under gravitational force - hence the name of the filter.
Pump fed pond filters
Sometimes it is not practical for one reason or another to gravity feed a filter system.
Then it will be necessary to use a pump fed filter system. Here the pump is situated in
the pond (submersible type) and it pumps water into the filter. It is very important here
to have free flowing medium, and a water bypass system fitted. This is because pumps when
situated in the pond tend to liquidise the solid debris thus making it very hard to
separate it from the water using settlement. The resultant high debris load in the water
can cause filter blockage and overflow - hence the by-pass.
Stage 1 of filtration - Settlement
Settlement of solid matter from the water can be achieved in several ways. The most
common are described here.
a) Settlement Chamber
This is a large chamber, where the solids are encouraged to settle out from the water
under the force of gravity. This works on a similar principle to a settlement lagoon
at a sewage farm. This should contain some brushes (one or two rows) to trap the lighter
floating debris such as blanket weed that will otherwise find it's way through. The
chamber should not be completely filled with brushes, or the natural settlement
action is impeded.
b) Vortex Chamber
Here the solids are removed by dynamic action that is induced by having a spiral
movement imparted to the water. The large solids are removed by the boundary layer
of water that builds up adjacent to the cylinder walls, and the central vortex column
removes the small solids. The most efficient vortexes are those that have no internal
pipes to impede the smooth flow of water.
Stage 2 of filtration
Biological filter
Once the solid matter has been separated from the water, the flow is directed to the
biological stage of the filter, and the action that takes place here has been described
earlier. The biological media can either be submerged, or can be configured as a trickle
filter "tower" where the media is exposed to the air, above pond level, and the water
is flowed through it via a spray-bar.
There are some ponds that still employ under-gravel filters, which are built under water
at the bottom of the pond. A (submersible) pump draws the water through them. These still
have their application in situations where garden space is restricted, but are no longer
highly regarded for a number of (mainly hygienic) reasons.
Stage 3 of filtration
Pumping
Within the pumping stage is incorporated not only the pump, but also all of the auxiliary
equipment that is fitted to the system such as UVS, magnets, Venturis, and sand filters
etc.
Of these devices:
UVC (Ultra Violet clarifiers)
Used to prevent green water by disrupting the algae at
cellular level. It causes the cells to stick together or flocculate, when they will settle
out and can be removed by hoovering, or are trapped in the settlement chamber where they
can be flushed away.
Magnets
By creating a flux in the water, magnets prevent the formation of filamentous
algae, or as we better know it blanket weed.
Electronic Devices
These will disrupt the calcium carbonate molecules in pond water, making
it unavailable to algae that forms blanket weed.
Venturis
These inject air into the return flows of water to the pond.
Sand Filters
These will remove the very fine particulate matter that can still left in the
water, giving it a final "polish" before it is returned to the pond.
Heaters
These can be included in the pumping stage for assisting in the over wintering
of Koi, or even for boosting top summer water temperatures which some of the
top Koi keepers now do.
Having read all of the foregoing, it can be seen that a filter is quite a complex part
of the Koi pond system, and must be regarded as a living component, just as the Koi
are. It should have equal care taken of it.
Maintenance
Already mentioned are flushes, which must be operated on a regular basis to keep the system
clear of sludge. No matter how good your settlement is, this always seems to build up to a
certain extent. You must take care also that no part of the filter system is cleaned with
tap water as this contains chlorine that will cause nitrifying bacteria to die back. If you
do have any parts of the system that do need to be washed, such as sponges, then do this
with pond water and don't get things too clean. Washing off the worst of the dirt is all that
is necessary.
Your filter must never be switched off, apart from the odd half hour for essential maintenance,
otherwise the nitrifying bacteria, starved of oxygenated water will again die back giving you
a less efficient filter when switched on again. Likewise UVCs (Ultra Violet clarifiers) must
never be switched off, apart from when medication of any sort is in the pond, although this
statement could be seen as controversial in some circles. If you switch off UVCs for more
than a short period, then you will get a build up of slime on the surface of the quartz tube
that will reduce its efficiency when switched on again. The unit will then have to be dismantled
to clean it with all the associated dangers of breaking the quartz tube. UV filament lamps
should also be changed once per season as the emission deteriorates after a while.
Filter medium
Filter medium is a subject that causes many an argument at club level, and there is so much
material available to read that it is very much "Hobson's Choice" as to which line you decide
to follow. Personally we would go for a larger sized medium, that would not tend to block.
Several types were mentioned earlier and this should make your personal choice somewhat easier.
A few brushes are seen as useful in settlement chambers, but we have also seen "Springflo" used
here, or even crumpled up fruit cage netting. Be careful not to fill your settlement chamber with
such medium. Leave enough open space for true gravity settlement to take place. What we would
suggest before making your choice of filter medium, is to take a look at what other hobbyists in
your area are using and emulate this. It is obvious that if a medium is widely used in one area
that considerable success has been achieved with it.
Matting cartridges are widely used, and many people swear by them, but sometimes they do take a
long time to go biological. The newer reticulated foam cartridges are more efficient.
Water testing
Water testing is perhaps the most important part of Koi keeping as this can tell you a lot about
whether or not your filter is working at peak efficiency.
a) You should monitor firstly ammonia, and the presence or not of this will confirm for you whether
or not your filter is performing satisfactorily, or whether it's efficiency is falling due to causes
that need urgent investigation.
b) You should then monitor nitrite (don't confuse this with nitrate), as nitrite appears after the
first stage in the oxidation of ammonia. You can have the situation where there is no ammonia, yet
nitrite persists, and this should lead you once again to investigate the filter performance.
c) The third parameter of importance is pH. This can tell you a lot of things about your pond, and
there is a certain band, which is better, suited to Koi. PH 7.5 to pH 8.3 is absolutely ideal, but
Koi can exist from pH 7.0 to pH 8.5 without harm as long as it is stable. Any reading outside this
range should be investigated and rectified without delay, but we would not advise that you tried
any alteration of pH by artificial means such as the widely sold "pH buffers". Not only would this
require a lot of buffering agent, but it would not be a permanent correction, would need to be
repeated often, and you could thereby be masking a filtration defect.
This Koi keeping hobby is, as you can see, rather a complex one but with a little learning applied
and a little common sense, we can hopefully surmount all of the difficulties that others fall into,
who are not prepared to acquire this knowledge and to apply it.
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Copyright Pets Parade Ltd. 2000