Solving The
Green Water Blues

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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 fertilizer.

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 evergreens.

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 cleaning harder.

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 property.

 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. limestone).
5. Fish, during the spawning season, are likely to be injured by the larger rocks. (I am taking the Koi Health Advisor course now).
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!

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