GARDEN FISH POND BIO FILTER
Hi - Welcome to my DIY Fish Pond Bio Filter Pages! :o)
I hope you find them useful.
What you get: In this
project I give some background into my pond, and the common
"green pea soup" algae problem (see Essential
Reading below), my personal opinion of pumps and filters
on the market and some modifications. I cover construction
of a DIY BIO-filter with a simple venturi system and vortex
chamber, and identify key issues in preventing algae,
and also provide some useful resources and forums for
I am not a professional, and my small pond
is not specifically for Koi fish, this project is purely
suggestions from my own experience and research. Some
aspects involve modifications to equipment that could
invalidate your warranty. I just enjoy trying things out,
and judging by the number of people looking for similar
information in fish pond forums, as one of them you might
be interested in my ideas.
Safety: Please be careful!
If you have a young family, do remember that kids are
fascinated by water. You really should think twice about
building a pond if you are unable to supervise youngsters
at all times. Tiny tots are fearless and don't understand
the dangers of water, and it only takes a minute for a
little person to drown, even in very shallow water. Having
said that, our grand-children visit regularly, and with
constant reminders they learn to be careful. But we never
let the youngest ones near the pond without an adult right
by their side.
Our pond, complete with lighting, and
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First things first
Before you get stuck into reading
about my bio-filter project you are probably asking
yourself "Is it worth
the effort?". In the last couple of years since adding the bio-filter my pond water
has remained crystal clear ALL year long, even
through the summer.
You have probably read some stuff elsewhere
about other people trying their hand at building a
bio-filter (a home for good bacteria to live and work),
and want some kind of reassurance. If you have a pond
without a bio-filter and you are experiencing green
water or fish health problems then you really should
I would also like to make it clear to you the importance
of having either a venturi or an airstone
to oxygenate the bio-filter. Oxygen is
the magic, the wow, the X-Factor in a pond. You need oxygen
to survive - your fish need it too! And your bio-filter
will be less efficient without a good supply of oxygen.
Also if green water is currently a big
problem for you then at night the algae will deplete the
oxygen in the water and your fish may suffocate! Green
algae in itself is not bad, fish like to eat algae of
various sorts, but when a bloom becomes excessive it will
compete with your fish for the oxygen. Photosynthesis
in plants (and algae) absorbs carbon dioxide and gives
out oxygen during daylight hours, but the process reverses
and uses up oxygen whilst producing carbon-dioxide at
night. This can cause distress for your fish.
There is a lot that goes on in
a ponds bio-cycle and your understanding of this will
greatly help you achieve miracles with your pond. I
will tell you about all this in due course, but I thought
I would just reproduce the following section here now,
which comes from right at the end of this project.
is my own proof and experience, but I have
also had several other visitors to my web site email
me how pleased
they are that once again they have clear, clean water
in their pond, using the simple principles explained
here, and without the need for expensive manufactured
The following taken from my Was it Worth It section....
"During the first 2 weeks of experimenting the
bio-filter didn't really get much chance to settle
down, what with
me pulling it apart every couple of days to re-arrange
the venturi. It is said that a bio-filter requires
about 6 to 8 weeks to mature, while the bacteria
and a balance is created in the ponds eco-system.
In the third week, with the early prototype venturis
not working very well, there was very little happening,
and while the water was beginning to look cleaner due
to solids filtration, it was still VERY green.
But in the fourth week, about 4 days after I got the
final venturi design working mixing air into the water
nicely, the water was starting to clear noticeably. Up
until now the fish could only be seen clearly if they
came within about 3 inches of the surface, any deeper
and they became a green blur. But after 6 days of oxygenating
the water going into the bio-filter the fish could be
seen progressively deeper, now down to 6 to 8 inches,
and the vivid green was weakening.
Now about 8 weeks after the whole bio-filter and
venturi setup has been fully operational we can once
our fish. The water is much clearer and cleaner. The
bio-filter has healthy looking gunk in it, and the
pre-filter does not seem to be blocking up as quickly
- I still
clean it regularly, but there is just not as much build-up
of muck in it."
So, are you sitting comfortably? Then let's begin .......
When we moved to a new house in 2001 we built a fish
pond. My wife didn't want anything too big, while I on
the other hand, had bigger ideas! Our garden pond was
to be a home for a few goldfish (not a large koi-pond
project), although as time has passed and the fish have
grown, it has started taking on some aspects of the koi-keeper's
When we were planning the layout we started by using
bricks on the lawn to mark out the shape and size. For
about a month we had fun battling out against each other
the actual size of the pond, which grew (me), then shrunk
(wife), every other day, until we agreed to differ.
This is the fruit of our labours after 3 years. At first
we just thought it would be mildly interesting, but in
this time we have grown to love our fish, and we get
many hours enjoyment maintaining and improving it, even
more so as our little fish have grown into quite size-able
We would even venture to say that from daily observation,
and getting to know them, that our fish actually have
different "characters". Maybe, as keen Scuba
Divers who have seen many wonderful and beautiful forms
of sea life, we have formed a respect and affinity with
our aquarian friends in their own little garden haven.
For great fish-keeping communities visit
my favourite forums at:-
They are very friendly and knowledgeable
groups of people who will make you feel very
welcome. There is tons of discussion going
on about fish of all kinds, problems whether
relating to the health of your fish or the
state of your pond, and advice on filters,
pumps and anything else you can think of!
In a Nutshell
The original plan was always to have a "terracotta
water urn" spouting water into the top of a small
stepped waterfall, leading into a little stream and "bog" area,
then into the main pond.
We didn't want a prefabricated waterfall. Many of the
fibre-glass or plastic waterfalls available were either
not interesting enough, the wrong shape for the layout
we wanted, or looked un-natural.
Most items used for our pond were bits and bobs that
we found in various places, and the millstone style edging
was a £50 batch lot picked up from the junk section
of a local stone merchants.
We used high-grade pond liner to line the pond dug into
clay (hard work by hand!), edged with substantial wooden
battens to prevent cave-ins, and hold the weight of the
millstone edging, and levelled using the old "water
in a hose-pipe with two funnels" trick.
View of the "urn" at the top
of the rockery, spouting into the waterfall, down the
stream, through the bog-area, and into the pond.
There are 3 levels:-
- Deepest at 3ft. has a Hozelock Cyprio 240v 4000litres/hour
fountain pump sat on a couple of bricks (now replaced
by a Titan 8000litres/hour on the bottom), which powers
a "water-globe" nozzle
(bell) fountain (which doesn't foul up as quickly/easily
a fine water-jet fountain), and also pumps
water up to the top of the waterfall (originally via
a Hozelock Bio-Force UVC Filter although this is no
longer connected) and my home-made "Skippy" style
BIO-filter (the construction of which is described
in this project).
- Middle at 2ft to give some depth below the streams
entry into the pond, so the fish can swim happily underneath,
and the shelf can hold mid-depth aquatic plants, plus
the shelf can be used to move the pump to a higher
level in winter to allow warmer water to stay at the
- Shallow shelf at about 9-12 inches around the pond-perimeter
for marginal aquatic plants (although we have discovered
to our distress that herons love this sort of depth
to stand and fish in!). We now have a slatted
wooden pond cover over the pond.
Building a Small Stream leading into the Pond
One of things that really makes a big difference to
the enjoyment of our pond is the little stream and bog
area. I have added some more information about this,
and so have created a new page on its own. Click here
to learn how to build
a stream leading into your pond.
It's well worth the effort. A stream and bog area attracts
so much wildlife; we get frogs, dragonflies, water-boatmen.
Also its a delight to see wagtails and blackbirds come
to wash in the stream. On occasions when heavy
level in the pond to overflowing, the smaller fish will
swim up the stream to explore.
During the second year our fish had babies, and the
adults were growing at an amazing pace. We started with
12 fish, a mix of golden orfe, sarasa comets, shebunkins, and ghost-koi,
all of which were about 3 inches or smaller. Now the
ghost-koi look fantastic and the biggest is about 12
inches long and looks very stocky and chunky. I'm sure
he'd make a good dinner (only joking).
For the first year the water was lovely and clear, even
in the summer, and we had good cover to provide shade
from the marginals, like lilies, water hyacinth, and "Fairy
Moss" - that red-leafed surface cover weed that
can take over the pond and you end up scooping out handfuls
of the stuff. Also in the boggy area of our stream we
had a good variety of bog-plants; Marsh Marigold, Horsetail
Rush, Water Musk.
The main pond, now covered with netting
since we lost 2 beautiful carp to a heron.
But, in the second year (Summer
2004) things suddenly started going wrong.
Three things seem to be the culprit:-
- The fish were much bigger, and eating more, were
therefore excreting more.
- Even though we put more of the Fairy Moss in the
pond to get more cover to keep out the sun, it didn't
survive, we think the fish were eating it(?).
- The yellow-flowered Water Musk had pretty much taken
over the entire bog-area, and so we had a mad
fit and removed ALL of it!
Ooops! Within about 2 weeks the water went a
pea-green, taken over by fine water-born algae.
We couldn't see below about 4 inches, any deeper and
the fish could only be made out as dark shapes passing
In hindsight we now realise that the plants
in the bog-area, with their massive root network,
were acting as a wonderful natural "veggy-filter" to
eat up nitrates in the water, hold back any crap
being pumped out of the pond coming back down the
stream, and provide a home for bacteria that was
helping the whole "balance" process.
When we designed our pond, waterfall, stream and bog-area,
we didn't really appreciate what a clever design we had
given "nature" to work with.
And we had just messed up by removing the most
vital part of the cycle. A natural vegetable biological
Initially we didn't figure out why it had happened.
At about the same time the fish had been traumatised
by the heron eating two of the larger fish, and we thought
the water was cloudy because they were stirring up the
bottom silt to "hide themselves".
I was also starting to get very fed up with cleaning
my Bio-Force filter literally every 2 to 3 days, and
having to remove the Cascade water pump from the pond
every week because that too was getting clogged up, mostly
by blanket weed.
I knew that sunlight was going to be a major cause of
the algal-bloom, but I was starting to get concerned
about the state of the water and the health of the fish,
the water at times was almost looking "brown/black",
and so started my quest to learn more about what was
going wrong and how to resolve the problem.
My web site has
a lot of useful information, but the one thing I
would most want you to gain from visiting this site,
is simply to understand life, the nitrification
cycle and why you need a bio-filter. For this you
should read the following section:-
Everywhere I looked there seemed to be plenty
of "remedies" in the form of expensive solutions and
additives which promised to eradicate the blanket
weed algae that was fast overcoming the pond, and
other remedies to remove the water-born algal bloom.
However I was already trying these to no avail.
It seemed like a threshold had been passed where
the algae was winning hands-down.
I had been toying with the idea of buying a new
bigger better pump, and getting a better filter
(although at this point had no idea what kind),
but all this seemed yet more expense given that
I had already upgraded from a Hozelock Cyprio 24v
Low-Voltage 2000litre/hour pump to the 240v 4000litre/hour
model earlier in the year, and also purchased the
Bio-Force UVC (Ultra-Violet Clarifier) 2200 Filter,
which now I didn't think was man enough for the
job (even though the performance ratings suggested
it should be sufficient).
I also felt that the Ultra-Violet clarifier was
just a gimmick because it didn't appear to be
any effect on removing the algal bloom, even though
my pond at about 400 gallons is by no means large
as compared to some peoples. [In fact 2 months
after finishing my bio-filter I had clear water
UVC bulb is no longer connected!]
So before making any impulse purchases I felt it
would be wise to do some research into the best
solution. I started doing some surfing around all
kinds of pond-related web sites, most of them obviously
trying to sell their stuff, and many of which didn't
appear to give the answers I was looking for.
Now before going any further into my project I
would like to show you a web site which I thought
was a breath of fresh air amongst all the
technological gobbledy-gook being pushed at me by
the myriad of web sites whose sole purpose appears
to be providing "good advice" with the
ultimate aim being that you buy one of the multitude
of different makes of pumps, filters and other accessories.
Skippy's is the site that gave me the inspiration
to have a go at building my own BIO-filter.
And the most wonderful thing is it explains all
about "freakin magic"!! You
NEED to know about Freakin
Magic in order to understand ponds (you'll
see what I mean if you don't already know), in fact
you'll learn about Nature and LIFE!
In fact even for experienced pond keepers, it makes
a great read - the emphasis is on helping you understand
how to work with Nature to create balance,
not against her.
Skippy's then tell you how to build your own DIY
vortex/settlement-chamber based BIO-filter. Its
a great site but for one thing. I don't think they
make it clear enough that you need to PRE-filter
the water going into the BIO-filter (maybe I missed
something!). For a novice like me I got the impression
their bio-filter was able to do everything. It wasn't
until after I had built my basic bio-filter based
upon their ideas, and then later read in their associated
forum about other newbies who similarly misunderstood
the need for PRE-filtering, that I made some changes.
(I also emailed them about it and they confirmed
I was right).
Well, this project is my own
take on the Skippy Bio-Filter. Once you've read their
web site (and I strongly recommend you do), if you are
interested come back and see how I've done it (and
implemented a simple venturi too - a mechanism to mix
air into the water with no moving parts), and also where
to get filter media for your Skippy filter if you're
in the UK (Skippys is a USA site).
So here it is ...... Skippy's bio-filter - take your
and absorbing these 3 essential pages (I find them fascinating and educational,
and have often gone back to them to remind myself what we are trying to achieve
when creating a bio-filter).
first by reading this
page, closely followed by this
page to learn about Freakin Magic, finally and probably most important is
this page about
you can go
page for their Bio-Filter construction.
Remember to come back here! :-)
Now I will show you what I have actually done,
and why. Click
here for the next stage of the project.
If you have any comments or suggestions about this project
please contact me:
And if you have found these web pages useful and interesting
and would like to make a donation, you may do so using
the Donations Page.
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GLOSSARY OF TERMS
Here is a short glossary of terms:
A biological filter is a vital part of any pond system.
This chamber (or chambers) follow the mechanical PRE-filter.
A good, long-established biological filter should be
home to a colony of "aerobic" bacteria that
convert toxic ammonia to less harmful nitrates. This
bacteria can be boosted with additions of fresh bacteria
culture, say at the start of the season, or after a new
bio-filter has been installed, and can be obtained from
any good fish pond stockist.
A venturi is a device that injects air into the pond
water. It is made very simply from pipework and has a
restrictor inside and an air tube which extends above
water level (you will see a diagram later on). Water
is pumped through the venturi, and the restrictor forces
air to be sucked in from the tube above water level,
and then mixes with water inside the venturi to cause
thousands of tiny bubbles that aerate the pond water.
It is used to improve aeration and movement of water
in the pond, and increase the oxygen level in the water.
A settlement chamber is the lower part of the filter,
to which waste is channeled. The solid matter sinks to
A vegetable filter containing plants like arrowheads,
water lettuce and water hyacinth, will absorb harmful
nitrates and phosphates from pond water.
Vortex: when this cone-shaped unit is installed, the
pipework from the bottom drain is connected to it.
When the valve is pulled, the water enters the chamber
in a whirlpool action drawing heavy solids down in
a gravity-fed, clockwise flow to the steeply-coned
base of the vortex. By pulling the vortex valve, these
solids are flushed to waste. The water exits the upper
part of the chamber via a pipe towards the next
stage of filtration.