The Nitrogen Cycle
The nitrogen cycle is the biogeochemical series of changes
in nitrogen and nitrogen-containing compounds taking place in
ecosystems. Fish consume and digest food, then excrete waste
in the form of ammonia. Their respiration absorbs oxygen and
releases carbon dioxide. Specialized species of aerobic bacteria
(Nitrosomonas) break down ammonia into nitrites (NO2-). Other varieties of aerobic bacteria (Nitrobacter)
process nitrites into nitrates (NO3-).
Aquatic plants (including some algae) photosynthesize nitrates
and carbon dioxide into food, releasing oxygen as a byproduct.
The process persists as long as a proper balance exists between
fish load, oxygen content and filtration capabilities. If one
factor goes out of balance, such as too many fish, the cycle
crashes. Pond design, stocking levels, water volume, turnover
rate and other factors put specific demands on a filtration system.
You should understand nitrogen cycle basics before designing
or installing your filtration system.
A good filtration system optimizes beneficial bacteria growth
to eliminate as much ammonia as possible. What filtration choices
do you have?
Filtration breaks down into two distinct categories. Mechanical
filtration removes physical debris; biological filtration eliminates
Mechanical filtration catches large particulates like leaves
down to fine debris suspended in the water. It does not change
the debris; it simply traps solid material. A good mechanical
filter clarifies water, prevents debris buildup and facilitates
proper conditions for biological filtration.
Biological without mechanical filtering is a recipe for disaster
and vice versa. Mechanical filtration alone cannot prevent toxic
ammonia and nitrites from reaching lethal levels. Conversely,
debris can soon clog a biological filter causing death to beneficial
bacteria colonies. Comprehensive filtration fulfills both functions.
Just as a pond consists of various components, the same applies
to filtration. A simple system includes a pump to move water,
a mechanical filter, a biological filter and a means of aerating
water via a fountain, waterfall or air pump. As the need arises,
add more components to create a filtration system suited to your
specific ecosystem and its needs.
The first line of defense in combating pond debris is a pre-filter.
Often found on small submersible pumps (addressed in my next
article), they consist of foam or other coarse sponge-like material.
Pre-filters keep pump intakes clear; they trap fine debris as
water passes through the filtering material.
Clean your pre-filters regularly to maintain effectiveness.
They prevent large debris from entering the water circulation
system. As pond owner, you do the actual work of emptying large
debris from the pond. Pre-filters prevent fish, small animals
and other large debris from entering intake pipes of external
In a small well-planted and maintained water garden, a submersible
pump with a pre-filter might handle the filtration duties. Debris
quantity increases in larger ponds; the burden then becomes too
much for a pre-filter alone. You may combine multiple pre-filters
or couple a pre-filter with an in-pond filter (performing both
mechanical and biological filtering tasks). However, these systems
have limits and require frequent cleaning to operate effectively.
When an ecosystem requires more than a pre-filter, skimmers
get the call. Skimmers remove floating debris from the water
surface and filter out large suspended particles. Available in
a multitude of shapes, sizes and designs, box skimmers and floating
circular skimmers are most popular. Both work by drawing water
into an opening to create a laminar flow on the waters
surface. This flow brings tree leaves, un-eaten fish food, grass
clippings and decaying plant material into the skimmer where
a net and filter pad capture the debris.
As skimmer design evolves, manufacturers add features striving
to build the ultimate pond skimmer. You can easily find a skimmer
with brushes or filters to capture smaller particles instead
of concentrating on large debris. New designs include UV sterilizers
and clarifiers. They assist bacteria colonization (usually in
the form of additional filtering material) to jump-start the
biological filtration process. Manufacturers advertise skimmers
capable of both mechanical and biological filtering for small
Skimmers perform mechanical filtration that assists biological
filtration elsewhere in the pond. Locating the submersible pump
out of the pond and into a skimmer bars debris from the pump
intake. In essence, the skimmer works as a super sized pre-filter.
Location greatly affects skimmer performance. Improperly located,
a box skimmer struggles to create surface flow, especially with
obstacles like plants or rocks in the path of the opening. I
like to place them opposite a waterfall (or other water return)
to take advantage of natural currents. Prevailing wind direction,
pond shape and other factors influence skimmer location. These
factors highlight the critical nature of a sound design to avoid
dead spots and sharp corners that trap floating debris.
Select your skimmer with a weir (opening) relative to the
dimensions of your pond. Too small an opening restricts pump
flow; too large an opening produces insufficient surface flow.
Most manufacturers offer multiple size weirs or an adjustable
weir you can tailor to any pond. Other features include provisions
for bottom drains, external pumps and automatic fill units. Choose
a sturdy unit built from strong materials. Soil pressure, especially
in areas with heavy clay soils or that experience freeze/thaw
conditions, can easily crush a flimsy box skimmer.
Pre-filters and skimmers address large floating debris and
other pump-clogging debris. However, they fail to capture all
debris before arriving at the pond bottom. Bottom drains exist
to accomplish this task. They act like vacuum cleaners for the
pond bottom. While capable of catching large debris missed by
pre-filters and skimmers, bottom drains primarily remove suspended
Unlike skimmers that remove highly oxygenated top water, bottom
drains remove poorly oxygenated bottom water. They draw water
down through the water column, mixing the entire pond volume.
This homogenizes oxygen content, moderates water temperature
and creates currents for playful fish.
Although a wonderful benefit, not all pond owners trust bottom
drains. Worries of leaks and clogs prevent them from installing
bottom drains. Like any other component, proper installation
leads to minimal mishaps. Overall, ponders encounter more leaks
from waterfalls and streams than from bottom drains. Perhaps
the nervousness of cutting a hole in a new liner is too much
for some people to bear.
In addition, bottom drains lack any benefit in stone- and
gravel-bottom ponds or in ponds without proper slope to direct
debris to the drains. They are problematic in cold weather locations
due to the danger of freezing, especially for ponds shut down
in winter. Avoid plumbing bottom drains directly to a pump, either
submersible or external.
In well-designed filtration systems, bottom drains flow into
either the skimmer or a settling chamber. The most effective
setups utilize gravity and waters habit of seeking equilibrium
to move pond water through the drain and into the skimmer or
settling chamber. Debate continues on which way to plumb drains
(skimmer vs. settling chamber). While both ways work, I favor
Combined with bottom drains, mechanical filtration takes a
quantum leap in effectiveness. Now filtration systems can reach
any debris in a pond. Ecosystems with small volumes or limited
fish loads may function well with a simple skimmer/bottom drain
setup. However, as volume and fish load increase, the amount
of fine debris may overwhelm the skimmer (which would then pass
the debris on to the biological filtration system). When conditions
warrant, add a settling chamber.
Their name comes from the process used to remove suspended
debris. Bottom drains feed water directly or via the skimmer
into the chamber. Once in the settling chamber, debris falls
out of suspension, settles to the bottom and exits by a drain.
Additional filter mats and brushes help trap debris. Then the
cleaned water flows into a return pipe back to the pond or continues
toward biological filtration.
The vortex settling chamber uses centrifugal force to remove
debris without mats or brushes. Circular movement of water within
the cone chamber forces debris to fall out of suspension to the
bottom. A drain allows removal of settled debris. You can plumb
together a series of vortex chambers to create a gradual debris
removal system. The last chamber removes the finest debris.
Pond owners contemplating settling chambers should prepare
for a significant outlay of space, time and cash. Settling chambers
require considerable volume (perhaps equal to 15% of total pond
volume). Housing a series of chambers requires a lot of space.
Plumbing complexity and maintenance increase with each chamber
While commercially produced chambers are expensive, the good
news is that a handy pond builder can design his/her own settling
chamber system. Serious koi keepers utilize a series of settling
chambers. The chambers themselves are uncomplicated. Plumbing
them and achieving the proper flow characteristics present a
Once you have an effective mechanical system, move on to biological
filtration. Sometimes the terminology seems intimidating. Once
you pass nitrification, aerobic and anaerobic environments, and
ion exchange, biological filtration works in a simple way. A
container houses a medium that beneficial bacteria colonize.
Water passes through or over the medium, bringing food to the
bacteria. They feed on ammonia and nitrites, converting
them into nitrates.
Many beginning pond owners falsely believe that clear water
always equals healthy water. You cannot see the danger that lurks
in your pond. Working on the unseen molecular level, biological
filtration destroys harmful ammonia and nitrites to make water
safe for live organisms. Signs of an effective biological filter
include healthy fish, sweet smelling water, thriving
plants and water with low levels of harmful contaminants.
Biological filters depend on an oxygen-rich environment free
of sediment and debris. If sediment and debris cover bacteria
colonies, they lose their oxygen and food supply, thus halting
the nitrogen cycle. This illustrates the vital nature of mechanical
filtration relative to pond health. Regular cleaning encourages
beneficial bacteria colonies and assists in avoiding a filter
crash. Too thorough a cleaning removes vital bacteria.
Lacking sufficient biological filtration, water quality degrades
and toxin levels elevate. This creates a potentially lethal environment
for fish due to oxygen deficiency and ammonia and nitrite poisoning.
If toxins alone do not cause mortality, the resulting stress
lowers immunity making fish susceptible to disease. Remember,
clear water is not always healthy water.
Biological filters exist in all shapes, styles and designs.
No perfect biological filter exists. Select your
filter based on specific factors such as volume, fish load and
pump capacity. Three basic biological filter designs prevail.
The simplest units, box filters, consist of a housing (the
box) with filtering pads and a medium like lava rock, plastic
balls or special ribbon. Water commonly moves up through the
medium and then spills out of the unit back into the pond via
a waterfall or stream. Filter pads help prevent debris from reaching
the medium, keeping the medium cleaner and functioning properly.
These filtering pads also give bacteria places to grow (when
Box filters are the cornerstone of most water garden filtration
systems. They work well despite their simplicity. They are worth
every penny spent given a reasonable fish load and a good design
with adequate mechanical filtration. A novice can easily set
up the pump/skimmer/box filter kit most manufacturers offer.
However, box filters have limitations. The problem is not
sub-par performance, but rather pond owners unrealistic
expectations. The double whammy of heavy fish load and chronic
maintenance neglect has sullied box filters reputation.
Reasons for this could consume another article. The problem is
not always the equipment, but more often how it is used.
When picking a box filter, avoid flimsy components with inferior
hardware. Seek housing strong enough to withstand pressure from
heavy clay soils without deforming it. Look for stainless steel
screws, nuts and bolts; the plumbing bulkheads and spillway should
utilize a mechanical seal to guarantee a watertight seal. I do
not recommend box filters that depend on silicone sealant to
Box filters should include provisions for a drain (to make
cleaning easier), be easily disguised and employ effective media.
With other factors equal, different media make one filter perform
better than another does. Choose a medium that cleans easily,
resists clogging, is relatively lightweight and has loads of
surface area. Even poorly designed box filters improve significantly
by replacing an obsolete medium like lava rock with a modern
Another type biological filter imitates the tried and true
swimming pool sand filter. Water enters an enclosed housing under
pressure (from the pump) that forces it through beads, plastic
tubes or other material. The pressurized water agitates the medium,
keeping sediment from accumulating and increasing contact between
the water and the bacteria. After passing through the medium,
filtered water exits and returns to the pond.
Found most often in aquaculture or as part of large koi ponds
with many stages of filtration, bead filters handle higher fish
loads than box filters. Costing more, they depend on well-designed
and properly installed mechanical filtration to function properly.
On average, they need weekly cleaning with back washing. Wastes
exit via a drain.
Each manufacturer has its own take on bead filters. Ignore
hype; look at construction quality and the features offered.
I recommend a drain, power back washing ability and combination
valving for easier operation. The type of medium plays a big
role. Avoid old media like sand. Ceramic beads, plastic tubes
and other patented media have taken sands place.
Due to their complex advanced nature, bead filters offer more
surface area for bacteria than box filters. Therefore, a bead
filter with the same footprint as a box filter typically outperforms
it. Cleaning does not require removing beads from the unit. This
minimizes disturbance of the bacteria, prevents damage from drying
out and avoids exposure to tap water while cleaning.
Bead filters suffer from some of the same shortcomings as
box filters. They require frequent maintenance to keep the medium
clean. Large debris can clog a bead filter. Low oxygen levels
can lead to the filter going anaerobic, causing a crash.
Moreover, the plumbing knowledge to install a bead filter is
complicated and they need a settling chamber. Bead filters represent
a significant step up in complexity compared to box filters.
For lack of a better term, hybrid filters represent the third
style of biological filter. These filters combine
mechanical filtration measured in microns along with biological
filtration in one unit. Agitating their medium with an oxygen
pump (instead of just passing water through them) defines next
generation filters. Sometimes called a fluid bed, this environment
creates ideal conditions for ammonia conversion using a very
small amount of medium. Manufacturers continually introduce better
next generation filters.
Bead filters depend on good mechanical filtration; next generation
filters require great mechanical filtration. A little debris
can clog their screens causing backups and flow problems. The
fluid bed is also problematic, sometimes collapsing (falling
out of suspension) which essentially stops whole process. As
with bead filters, the plumbing complexity is greater and may
be beyond the skills of a beginner.
The high price of next generation filters places them out
of reach for most ponders. However, they clearly show where filtration
is going. As technology improves and prices drop, more pond owners
will employ this advanced technology.
UV Sterilizers and Clarifiers
UV sterilizers and clarifiers work with the filtration system
to help control single cell algae and microorganisms. UV clarifiers
kill primarily algae while sterilizers destroy bacteria, other
disease-causing organisms and single cell algae.
Do not view ultra violet units as filtration units. However,
when incorporated into a well-designed system, UV clarifiers
and sterilizers offer increased protection against green water
and fish diseases. Always follow manufacturers safety directions;
keep them clean to maximize performance.
UV bulb effectiveness degrades over time. Replace your UV
bulb yearly even if it still produces light. Avoid low wattage
units except for very small water gardens. Improved designs replace
glass bulbs or sleeves with acrylic or other shatterproof materials.
No article on filtration would be complete without addressing
natural filtration. Consider wetlands, bogs and marshes to appreciate
how well nature filters its water. The ground acts as a mechanical
filter. The same bacteria found in biological filters abound
in gravel streambeds, plant roots and the soil itself. Water
gardeners imitate this natural filtration method with a bog or
well planted stream.
I see manufactured filtration products pushing natural filtration
into the background. Koi ponds typically eliminate natural elements
such as aquatic plants due to the mess inquisitive koi create.
Some people downplay the role of plants, especially submerged
plants like anacharis, in creating well-balanced water gardens.
Over-reliance on technology is shoving pond hobbyists away from
what they endeavor to copy . . . a natural ecosystem.
Constant upgrades and improvements in filtration equipment
highlight our evolving understanding of nature. We do know that
natural systems like wetlands cleanse untold tons of water every
year. These systems rely on interaction between plants, bacteria,
soil and organisms to accomplish that task. By paying attention
to natures model, our own ponds can benefit from natural
filtration -- even something as simple as using a few water hyacinths.
Natural filtration in the water garden or koi pond does not
have to be elaborate. Vegetative filters are widely used as a
pre-filter and settling chamber combined. Root systems of plants
like irises efficiently absorb nitrates and remove debris from
water. Water hyacinths make extremely effective filters. Some
municipalities use them to treat sewage. Submerged plants (oxygenators)
excel in drawing fine sediment out of suspension and adding oxygen
to the water.
Natural filtration typically entails planting a portion of
the pond (or a nearby area) with bog plants. Accomplish this
with an area saturated by pond water, or perhaps has a stream
or other water source flowing into it. The bog provides interaction
between its plants and the pond water. The plants absorb nutrients
(nitrates) and capture debris. Many small water gardens use no
filtration method except natural filtration.
Natural filtration is giving way to manufactured filtration
systems because of maintenance and seasonality. You must prune
and thin bog plants to sustain their vigor. Many aquatic plants
are invasive, so bogs can quickly get out of control if left
untended. Additionally, plants in non-tropical regions decline
in the fall and revive in the spring. This leaves months of the
year with reduced means of removing impurities from the water.
Since filtration systems represent the largest time investment
pond owners make in their water garden or koi pond, many prefer
cleaning a filter weekly to caring for a bog. This becomes a
simple decision of economics for most owners. While this choice
is understandable, ponders should consider any natural means
of enhancing their pond with aquatic plants. The rewards are
measurable in water quality improvements and the beauty of the
In summary, filtration systems present a series of tradeoffs
between time, cost and performance. If cost and space were irrelevant,
the ultimate low maintenance system utilizing the latest technology
would be everyones choice. This perfect system
would still require maintenance; that leaves you making choices
about your own filtration system.
How much work are you willing to do? How much are you willing
to spend to reduce the workload? How perfect an environment do
you want for your pond inhabitants?
If you grow show koi, your system will be quite different
from a water garden with a few goldfish. Like all other parts
of a water garden or koi pond, your overall design plan guides
filtration choices. Avoid the temptation to skimp on filtration
and routine maintenance. After all, it makes little sense to
create a beautiful oasis for magnificent koi if you only see
them when they surface to gasp for oxygen.
* Mechanical filters attract beneficial bacteria and perform
some biological advantage. Biological filters strain water passing
through them, contributing some mechanical filtration help.