July 17 2012 12:42 AM

What causes it and what to do about it

Why is the water green?” you might ask. Whenever there is a surplus of available plant food (nitrate), Mother Nature steps in and supplies something to use it, in an attempt to achieve balance. Your water is green because of an explosive growth of small algae and phyto- and zooplanktons.

Balance is the key to clean, clear water. With normal pond water pH parameters (6.4 to 8.4), there should be enough regular plants to take up the excess nutrients. That is usually all that’s required to achieve balance.

Algae love the higher pH, but even in a balanced pond, algae are still there. If they’re kept under control, they won’t multiply. If the water is not balanced, algae will multiply quickly making the water “green”. These small nutrient consumers can triple their numbers in one day during summer-type conditions.

So you’ve got a case of Syndrome; but cheer up, it’s not as bad as you think. With a little thought, education and detective work, your obviously complete am monia-nitrate cycle can be put to use to give you and your customer clear water.

There are solutions to clearing up the problem. Chemicals are not the only answer, and many times it’s not necessarily the best either. Many pond “chemicals” more often than not cause more problems than they solve.

The best solution is to have some plants growing in the pond to consume this food. Lillies, marginal, and floating plants are ideal. They will starve the green stuff back to its balance point. If you can’t have or don’t want plants in your pond, there are still some options open to you. They will be more limited.

You may need to do large water changes on a weekly basis in order to dilute the level of nitrates in your pond, much like the water changes commonly recommended to aquarium owners. You might note that doing a 50 percent water change weekly on a 100-gallon aquarium is a lot less water consumption than doing a 50 percent water change on a 2000 gallon pond.

Adding a plant pond area outside of your pond is a great way to add color and plants, especially if you have koi that are destroying any plants you introduce to the pond. Check out www.bogfiltration.com for some great ideas on how to add a plant element to your pond, as well as additional filtration and polishing of your waterscape’s water.

Things to check 

Overfeeding fish

Your customer might possible have been feeding his fish too much. Think of each handful of food as being a handful of fertilizer thrown out on your lawn. The more fertilizer, the more lawn it needs to be spread over. Excessive fish food equals excessive fertilizer and not enough lawn to spread it over. Without enough plants to help balance the water, your client’s pond will turn green.

With a green water problem, the recommendation would be to stop or radically reduce the amount of food given to fish until you’ve reached balance. Once it’s balanced, you can slowly increase your food amounts.

You may not be able to return to pre-balance levels if the water starts to green up again, but you will find your balance point usually within 10 to 14 days after you stop feeding the fish, and by adding sufficient plants to make up the difference. The fish will not starve to death in this amount of time, and will benefit tremendously from the clearer water by having more oxygen available for them to consume.

Too many fish

If you have too many fish in too small an environment, you may have to thin them out, or enlarge the environment to handle all the fertilizer they produce. In the oceans or lakes, they are spread out over thousands of cubic feet of water per fish. With good circulation and filtration of a pond, we can get away with a denser population, within certain limits.

You should remember, the more fish, the more oxygen they require, thus more frequently circulated and oxygenated water. A waterfall or fountain nozzle radically increases the amount of available oxygen in water versus uncirculated, stagnant water that has a low level of oxygen.

There is still a balance point involved. Once crossed, bad things start to happen to your fish and to the water quality and clarity, not to mention your enjoyment of the pond and fish.

Not feeding fish and the water is still green

If you’re not feeding the fish, but the water is green, there are either too many fish, or not enough plants to achieve balance. Also think about the circulation rate of the pump compared to the volume of water the pond holds.

With a pond under 5,000 gallons, the pump should be moving the total volume of water of the entire pond (called “turnover”) through a filter and over a waterfall or through a fountain nozzle at least one or more times per hour. The smaller the pond, the more frequently you can economically “turn over” the water. This keeps the oxygen levels high, thus making it easier to achieve balance.

No fish and still green water

Not having fish in a pond doesn’t mean the fertilizer effect isn’t still going to happen. Any organics, like tree leaves, plant stems and dead flowers all start to break down the second they die. This is part of the circle of life.

As they break down, they eventually convert into, you guessed it, plant food. Having good circulation and adding oxygen to the water will help the existing plants optimize their nutrient intake, as well as possibly adding even more plants to finally achieve balance.

Beneficial bacterias

Adding a quality bacterial product blended to eat and convert these organics can’t hurt, and usually helps. A liquid blend will act faster than a powdered form, which is usually a freeze-dried blend. Liquid goes to work immediately, while powder takes three to five days to get “resurrected.” Powder has a longer shelf life. Liquid should be used with-in a year of packaging, not purchase, in my opinion.

UV clarifiers

Some people swear by ultra violet clarifiers and claim that you’ll never have clear water without one. I respectfully disagree. With balance (plants moving, oxygenated water beneficial bacteria and adequate filtration) you’ll have clear water almost every time.

Green water only occurs in an unbalanced pond. A UV clarifier might help clean up a green water problem initially, but in the long term, without balance, it’s just a band-aid or crutch. Learn and establish balance and Mother Nature gives you that clear water for free.

UV clarifiers (the term sterilizer is often incorrectly used, in my opinion) will damage the cell wall of the green water, by causing organisms to pass through its exposure chamber. It damages the organisms passing through it enough to prohibit their ability to reproduce, causing them to eventually mature and die, without reproducing. It will not touch string or mop algae, since they don’t pass through the reaction chamber.

Also know that you can’t pump the water through too fast, and the bulb must be new enough to maintain the proper spectrum of light necessary to do this. A word of caution, bulbs do degrade. Even if still glowing, after as little as six to eight months, it may no longer be giving off light in the proper spectrum to do its job. This necessitates a bulb change every year.

The downside is, if not trapped in a proper filtration system that is frequently cleaned, the dead algae lay there and rapidly decompose, releasing their nutrients into the water to feed more algae. Thus string or mop algae may pop up, or it will create a rapidly degrading water quality. While the water may possibly be clear, it could still be sucking oxygen out of the water faster than you can replace it.

I prefer balance for a pond. Although I feel that UVs do have their place in specialized aquaculture, it may not always work in a properly balanced and maintained pond or water garden. A UV clarifier could definitely be used to assist with balance, but should not take its place.


These are the primary clues and reasons for classic Green Water Syndrome. I hope this information helps you resolve your customers’ problems, and as a result, increase their enjoyment of their pond or water garden. It will also enhance and reinforce your credibility as a professional.

Remember, it takes time to achieve balance. The hardest thing to teach many pond owners in today’s instant gratification society is to be patient and let nature and your knowledge do the job. Too many people start throwing everything up to and including the kitchen sink into their pond to combat an issue that is better served with knowledge and understanding. Many times during the course of the year, I advise people to stop throwing chemicals into their pond before it spontaneously combusts, or more likely kills all their fish at the first sign of ignition.

Editor’s Note: Dave Jones is the owner of The Pond Professional.