When I was asked to write about what
it takes to have a crystal clear pond,
a lot of ideas came to mind. This is a
topic I could easily write an entire book on and
still only scratch the surface. So instead of glossing
over all the factors contributing to a crystal
clear pond, I will focus on the most important
There are several different types of filtration,
but the two that will have a major impact on the
clarity of your water are mechanical and biological.
While both of these filtration types can be
man-made or naturally made, I am going to
cover man-made filtration specifically.
Of course, just because we are building the
filter doesn’t mean that we can’t use natural
materials for the media. Thus, I’ve chosen to
write in detail about a natural media that, in a
lot of circles, may be considered a dirty word:
rock and gravel.
## Rock and Gravel ##
A very old type of filtration media,
rock and gravel were used for a long time
but have lost favor with many ponders.
One of the reasons for the falloff is that
the surface area per cubic foot is not very
high — or so people think. Yes, rock is a
solid material that takes up a lot of space,
but it is also a natural material that’s
formed on a very fine structure and then
is eroded on a microscopic level. With
that in mind, the surface would have
a fairly high microscopic surface area,
which is never included when talking
about gravel surface area.
Even taking that into consideration,
a filter using rock or gravel will require
a larger footprint to handle the same
size pond as some of today’s newer
medias. But the smaller the filter, the
more frequently it requires maintenance.
Thus, the small amount of maintenance
required to maintain many rock or stone
filters is not easily accomplished with
Rocks on the Bottom
When we talk about putting rock and stone on the bottom of the pond, we immediately stir up passionate feelings in some people. There is a lot of debate about
whether a pond should have stone on the bottom, and the argument boils down to the buildup of debris in the gravel.
To speak to this issue, let me tell you about a pond that I am very familiar with
— a pond I built 22 years ago! This pond is six feet deep and has about six inches
of gravel on the bottom that has never been cleaned.
Now, I know what you’re thinking: “There must be a whole lot of debris in that gravel.” But when I swam in the pond last year, I dove to the bottom and dug in the gravel … and there was no debris to be found! How is that accomplished? The secret is an undergravel suction grid system. If designed correctly, it performs
excellently with very little maintenance.
Undergravel Suction on a Small Scale
What is an undergravel suction grid filter? Well, most of you are familiar with its smaller counterpart: the undergravel suction filter in aquariums. A filter like this is built with thin slots in the plate, which is supported off the bottom of the aquarium. Small gravel is placed on top of this plate, and water is sucked through
the gravel and the slots.These filters work great for a while, but then they need to have a lot of maintenance done or they fail. By “maintenance” I mean the gravel on top of the filter suction plate has to be cleaned.
Anyone who has done this maintenance on his fish tank knows it is not a funchore. In a busy aquarium full of life, waste and debris quickly build up in the
slots, causing the gravel to clog up faster than you can (or want to) clean it.
Ponds vs. Aquariums
If we want this type of filter to work in a pond, we have to look at why it has problems in an aquarium.
The real difference between the undergravel grid in a pond versus in an aquarium is the space in the grid. In the aquarium, the space between the pieces of gravel and in the slots in the suction plate is very small compared to the waste produced. In order for the system to allow water to flow through and not clog up, the waste has to be
almost completely eaten by bacteria. This process takes longer than it takes for the gravel to clog up. Therefore, the filtration simply cannot keep up with the waste. The filter clogs up and fails.
To avoid this problem in the pond I
built, I designed the undergravel filter to
have a series of pipes on the bottom with
⅜-inch diameter holes drilled in them.
The pipes were buried in .- to 1-inch
round gravel with about two inches above
the pipe. The spacing between the pieces
of gravel is fairly large, and the holes in
the piping are large compared to the
waste to be broken down. All the holes
in the suction pipes are six inches apart,
providing the waste a lot of area to fill.
In this grid there are six inches in
every direction that would have to clog
up before this filter would need to be cleaned on every suction line. Of course,
if this filter were only a small part of the
bottom of the pond, then there could or
would be enough waste to clog the grid
before the bacteria could break it down
enough to get rid of it. Therefore, the
larger the area of the pond’s bottom that
can be part of this filter, the better.
Will this filter ever clog and need
cleaning? The answer is yes, but the better
question is: How long will it take? There
is no set answer. It all depends on how
much debris or waste is being put into the
pond (or being made by the pond).
## Success Story ##
Earlier I wrote a little about the 22-year-old pond with a gravel bed that has never been cleaned. This happens
to be my own koi pond and my design. Located in Batavia, Ill. (about 35 miles west of Chicago), this pond was built to
be as maintenance-free as I could make it.
No, it is not completely maintenance-free
… but it is close. It is about 18,000
gallons and requires an average of three
minutes of maintenance a week. None of
the maintenance is spent on the undergravel
suction grid filter. The pond has
never been emptied or cleaned since it was
built. Based on what I have seen, the filter
will not clog up as long as I am alive or as
long as the liner lasts. My guess is that the
liner will last for another 25 years.
I did make a mistake when I designed
and built this pond. I used .- to 1-inch
round limestone gravel. The problem is
that after 22 years, the limestone gravel
is shrinking in size. I believe I may have
to remove this gravel and replace it with
gravel that takes longer to erode.
Of course, I never thought that the
filter would go this long with no maintenance
required. Sometimes you stumble
on the right combination of ideas and
designs and things work far better than
expected! Is it working in Illinois because
of the climate but possibly would not work
elsewhere? I would say no; I just returned
from California, where I saw a pond that’s
about six years old and has one of these
filters. It is over-stocked with koi, and they
eat well. But the grid is working great and
has not been cleaned. I saw this pond two
and a half years ago and it looked good
then — but it looks even better now.
Will this design work in every situation?
That I can’t answer, because this
type of filter hasn’t been used in every
possible circumstance there is. But it has
worked perfectly every time that I know
of it being tried!
I have also used this system as a pressure
undergravel grid filter, and it has
performed perfectly for the last seven
years. The only complaint from that
customer is that the water is too clear. A
suction undergravel grid filter normally
uses an external pump or air lift system
to run it, but a suction filter can also use a
submersible pump. A pressure undergravel
grid is built similarly to the suction grid,
but water is pumped through the grid.
The pond that is pictured above uses both
suction and pressure undergravel grid
filtration. The picture was taken when the
pond was five years old. Both systems are
being run off the same submersible pump.
This is a very formal pond and we kept
everything inside the pond.
Versatile and (Almost) Maintenance-Free
In conclusion, the undergravel suction type of filter I’ve described will give you a
great mechanical and biological filter. It is hidden in the pond and doesn’t require an area larger than the pond. If done correctly, it has proved to require little, if any, maintenance.
It can be used with either external or submersible pumps. It also works great with the latest air lift technology. I don’t know if “the perfect filtration system” really exists. But this one comes close enough for me.