Green Water Blues
Click images to enlarge
Joyce Van Kirk, who lives near Des Moines, Iowa, wrote
our email discussion
list with a problem -- green water. Since it is something
that afflicts many pondkeepers, we pass along the questions and
many of the answers, in the order received.
In 1999 we hired a guy from a nearby university to come and
design a pond for us (above). It is somewhere in the neighborhood
of 25,000 to 30, 000 gallons. He put in a pond, waterfall and
a bog. The bog is maybe 15' by 10' and is filled with pea gravel
and bog plants. The water is supposed to circulate through it
and act as our filtration system. The actual pond liner has no
rocks on the bottom, just the sides. The first year it worked
fine. After that the bog has plugged with sediment and it has
become a nightmare. I wouldn't know if we had fish any more because
I can't see anything. Just green water. This year he (the original
designer) came back with an installer to try to resolve the problem.
They felt that if we put in two Grande Skimmers on each side
and used treatments of Super-charged Flocculant and Dry Bacteria
we could get it corrected. We did and it didn't work. The installer,
behind the back of the designer said he thinks we need two Grande
Biofalls installed and with the little help from the bog he thinks
that might solve the problem. The installer also believes that
it was a mistake not to have covered the pond liner floor with
rocks. The designer doesn't like rock on the floor of a pond.
Hard to clean, he says. They disagree. I am inclined to think
the installer is right and that in some respects the pond is
much like an aquarium. You must achieve a balance just as in
an aquarium. I am beginning to feel strongly about the rocks
in the bottom and the addition of the 2 Grande Biofalls. Would
some of you offer comments?
Kit & Ben Knotts, Florida
We say fire them both!!!!!!! We are believers in natural balance
and this is achieved by doing less rather than more. If your
bog is clogged, bypass it in your recirculation system for now.
Be sure your pump is not on the bottom. ADD MORE PLANTS! For
this time of year you need at least 70% coverage of the pond
to achieve balance. If water hyacinths are legal in your state,
add a pickup truck load. Be sure to anchor them to the side of
the pond (they're floaters but don't like to float around). In
a week or two your pond will be clear.
Rich Sacher, Louisiana
In trying to help you with your algae problem, there is more
information needed; for example, your pond is located at the
bottom of a hill...does rainwater flow down the hill and into
the pond? If so, organic matter and fertilizer will enter the
pond every time it rains, and cause algae to grow in the water.
Even if the grass on the hill is not fertilized, the rainwater
will add nutrients to your pond. There should be a berm behind
your pond big enough to divert all rainwater around the pond.
Also, what kind of fish do you have in the pond? How big and
how many? If there are a large number of fish, especially big
ones, and you are feeding them daily (or twice a day), that too
will add to the algae problem. You did not mention if you have
under water grasses, which are so effective at absorbing nutrients
and eliminating algae; if you have koi, they will eat the grasses,
and so instead of grasses, you may need a biological filter to
get clear water. If you only have goldfish, they are compatible
with underwater grasses (like anacharis or cabomba) and you should
stop feeding the fish, and add several hundred bunches of anacharis
or cabomba to the pond. There is enough material in any big pond
to keep goldfish happy without adding food...which is like adding
If the bog was designed correctly, even if the gravel is clogged,
you ought to be able to pass some water from the pond onto the
surface of the bog, toward the back, and have it flow by gravity
down into the pond. This presumes the bog was done correctly,
with its own liner, and at enough of an elevation that gravity
will bring the water back to the pond. The amount of water should
not be a torrent...a trickle is more effective.
Lifting the pump off the bottom of the pond helps prevent recirculating
debris which would otherwise settle; adding water hyacinth or
water cabbage, along with another big water lily or two, will
give more surface coverage and also help clear the water.
Personally, I do not like pebbles or rocks on the pond bottom...too
complicated when it comes time to clean the pond...but other
people will swear by them (Maybe the pebble people will help
clean your pond when it needs it...)
Josh Spece, near Independence, Iowa
Nice to see another Iowa ponder and Victoria grower on the
list!! Your pond and Vics look wonderful!! The main problem that
I can see is a lack of plants in your pond. Getting more shade
on the water will help as will plants that compete with the algae
for nutrients. Water hyacinths and water lettuce are perfectly
legal here in Iowa, so if you can find a bunch, get them.
Josh's Victoria in his newly remodelled big pond
I definitely echo the no rocks on the bottom of the
pond. Within a year or two, your entire pond will be in the same
boat as your bog filter ... clogged with decaying matter and
an excellent food source for algae. Very labor intensive to clean.
What size of pump are you running? With such a huge pond,
I wonder if you are getting adequate circulation ...
Josh's lotus 'The Queen' in another pond
Jamie Vande, Germany
My cousin's pond had the same trouble the second year after
I designed it. The problem is essentially an overdose of sun
caused by a lack of plant cover. As mentioned, 70% coverage is
an excellent rule of thumb. It's the overheating of the water
which causes the algae bloom (actually, a case of too much energy
entering the system). You are right on the mark with the aquarium
comparison, a balance is required. I would add something to the
bottom, be it rocks, gravel or even soil (my least favourite)
and plant it with "oxygenating" plants, such as Myrophyllum,
Elodea, etc. These plants will quickly assimilate excess nitrogen
in the water and can then be partially harvested to remove it
from the system (great for compost or mulch, by the way!). Water
hyacinth, water lettuce, etc. will perform a similar function
while giving some sun cover. From what I can see in the picture,
you simply do not have enough higher plant life forms to keep
the system in balance. I would add a small grove of Nelumbo nucifera,
perhaps some osmunda ferns as islands near a bank (wonderful
effect!) and a few hardy Nymphaea to round it out. I would, also,
plant a tree to shade part of the pond from the hottest sun.
There are many smaller, decorative maples, Paulownias are extremely
graceful, Magnolias as well, just stay away from willows and
My own experience has shown me that, despite all the wonderful
equipment for ponds, such as filters, pumps, scarecrows, lighting,
et al, Mother Nature is still the number one pondkeeper and trying
to imitate (or is that honour?) her complexity brings the lasting
success. Given time, your Victorias would probably solve the
problem as they develop, it's just slower.
Dan Dixon, Tennessee
Your pond looks very natural to me. That said, the balance
you want is not really a "natural" one (most natural
ponds ARE green :-) ), since you want your water to be as clear
as possible. Rocks on the bottom might provide a bit of extra
surface area for beneficial bacteria, but they won't really contribute
much to any sort of balance that will give you clear water. On
that point, the designer is correct in that it will just make
The green water is due to high amounts of nutrients in the
water column, probably due to all the decomposing sediment in
the bog, plus fish waste, whatever leaches out of pots, organic
matter coming in with rainwater, uneaten fish food, etc. Normally
green algae clears up on its own, but you probably have so much
organic matter in there that the nutrients in the water column
are constantly replenished.
I would clean all the sediment out of the bog and put some
kind of a prefilter in front of it so that sediment is removed
from the pond entirely. Also clean the bottom of the main pond,
then do a partial water change. Remove the gravel in the bog
that traps sediment. Replant the bog as heavily as you can, including
some fast-growing emergent species like parrot's feather, pennywort,
pickerel, etc. to absorb nutrients. Use baskets that allow the
roots to spread. Put hyacinth, lettuce, salvenia, ambulia, egeria
or other such fast-growing plants that float in the main pond
to help absorb nutrients and deprive the algae of light. Feed
fish VERY sparingly. Eventually the plants will out-compete the
algae and the water will clear. Once the balance is restored,
you can thin out the plants you don't want.
Optionally, a UV sterilizer of sufficient capacity would bring
the green algae under control quickly, but the actual problem
(too much decaying organic matter) will not be eliminated.
James Horne, Canada
I'm planning something similar for my pond and I think that
the system would have been much farther ahead if you had used
a wetland marsh as the biofilter system, and populated it with
cattails and water hyacinths and the like.
Was the water run through the bog all winter long? If not I'd
guess that the real problem was that the "good" bacteria
died off in the bog from lack of flow and that now it doesn't
function to breakdown the waste properly so the algae are getting
to use it as nutrients instead. Green water really isn't bad
from a health of the pond point of view though. Its up to you
of course, but I'd probably not install any new and expensive
bits without a better understanding of what's really going on
in the ecological system sense.
Have either the Designer or Installer actually done any water
tests? Taken samples? Checked nutrient levels etc.? Grande Skimmers
and Grande Biofalls act exactly the same way. They are glorified
high tech versions of what you already have, a biofilter with
media and particulate removal, so putting two falls in isn't
likely to solve the problem any more than the skimmers did, and
unless you learn what caused the problem you're likely to have
it again next year regardless. I'd really lean towards doing
some water tests and perhaps digging out some of the bog to have
a smallish water hyacinth pond at the inlet as a start, and to
replace the gravel. Don't go too fine on the gravel in the bog
area. Small rounded gravel like pea gravel is about the worst
possible choice. Best would be crushed lava rock 1.5"-2"
size as it will have much larger surface area to grow bacteria
on and be less prone to clogging.
Also its really important to have adequate volume flow of water
past the bacteria in their living sites to maintain them in a
healthy state. As was said, a "natural pond" has many
many things that all need to balance, and that tends to take
some time and a few cycles. You may have to make some adjustments.
Dick & Pam Beal, Florida
We don't know of any good, sound reason to have rocks on the
bottom, some of our customers have tried it and removed them.
Just something to stumble over when wading in your pond.
Green water usually that bothers you much more than your fish.
How often do you feed them and how long does it take them to
eat it? We've encountered situations where grandkids were slinging
handfuls into the pond.
We agree on achieving balance. We are in Florida, with lots of
rain and heat. Many of our customers with liner ponds do not
have pumps, filters etc. just lots of plants, limited fish and
they experience clear water year round. Do not forget the underwater
plants, anacharis, mare's tail etc., etc. Duckweed helps plus
the fish eat it and water lettuce (which we're prohibited in
having by state law) is excellent, ditto hyacinth also banned
in Florida. How much? A few sprigs isn't going to matter, a few
bushels might. I subscribe to Kit's truckload suggestion for
your size pond.
From your picture it appears your pond is at the bottom of a
slope. Does run-off enter the pond or bog? We've experienced
many problems with run-off carrying lawn fertilizer into customers'
ponds after they'd located their pond on the lowest spot on their
Linda Siler, Missouri
To add to the comments about rocks in the bottom of a pond,
I ask people if they remember shag carpeting. It was pretty on
top but nasty on the bottom. Rocks or pea-gravel in the bottom
of a pond will do the same over a period of time; debris, dust,
pollen, seeds will settle in the cracks and crevices of the gravel
and cause anaerobic problems. In aquariums this no problem because
they are inside the home, but outdoors we are at Mother Nature's
whims. I know people who own garden pond maintenance companies
and they will not clean ponds with gravel in the bottom. If the
pond owner must have the gravel then mortar it in. The old timers
who have been installing ponds forever will not put loose gravel
in the bottom of ponds and pure koi lovers will not have gravel
in their ponds. Some installers will also plant lilies directly
in the gravel and, talk about a nightmare! As we all know some
lilies are such aggressive growers that the tubers can run the
length of some ponds.
Donna Fish, Illinois
Although many have made great suggestions about your situation,
I would like to comment about northern ponds. Watch out for too
many underwater plants. Some are great, but during the winter
if your pond is snow covered, they take the oxygen from the water
and your fish.
I would take care if you decide to revamp your connected bog.
If you have a lot of undesirables in the rocks and start disturbing
them, they could drain directly into your pond unless you somehow
block it while doing this. I would only dig out one well establish
plant and smell it. If the root system/soil smells rancid, best
to block the flow back to your pond while cleaning it out.
Green water is really not harmful to anyone but the pond owner
who wants to see to the bottom.:)
Shelly Klinger, Illinois
I'm a firm believer in NO rocks.
1. It displace precious water that could be better used to house
more fish or plants.
2. Rocks make it dangerous to actually get into your pond &
walk around (with more of a chance for a puncture to occur).
3. A GREAT amount is added to the cost of construction if rocks
are added to the bottom.
4. Certain rocks can react to the water, upsetting the pH (i.e.
5. Fish, during the spawning season, are likely to be injured
by the larger rocks. (I am taking the Koi Health Advisor course
6. Mulm (i.e. fish waste, etc.) gets down between the rocks and
actually harbors the "bad" bacteria, parasites, etc.
I've cleaned off the rocks from the bottom of a pond a year old
and found a 3" deep layer of mulm. The poor fish are swimming
in their own toilet, even though the water appears clear!
7. All rocks develop a natural algae coating, which masks the
beautiful granite that you paid so much for.
8. The colors of the koi and plants are complimented by a plain
black (or blue) liner as the background.
9. Lilypads, floaters, and marginals help to give the required
60-70% water coverage for the surface of the pond. You may see
their containers in the early spring, before the water warms
up, but they are soon covered by their leaves. I have both koi
and plants in my ponds and have learned that if I feed them watermelon
(just float a chunk of it, rind and all, in the water and watch
them go to town!) they leave my plants alone.
10. It's harder to use correct measurements when treating fish
for parasites, etc., when you have rocks in the pond because
of the displacement of water. Too much medication will wipe out
an entire population of your "babies". Too little will
do no good. In other words, with rocks in the bottom the standard
formula for finding the gallonage of your pond is totally off.
Every book has a different rule to live by, just as all ponders
have their own experiences to share. What works for one may not
work for another. The ability to read about the experiences of
ponders from so many parts of the world, learning from their
successes and mistakes, really makes the water gardening life
a never-ending education!