Are Dead Zones Killing Your Fish?

What makes water gardens and fish ponds look beautiful and inviting? That’s easy: clear water and healthy fish and plants, combined with good design, all  integrated into an attractive landscape!

This dead zone area behind the stepping stone path can become more of a living zone by adding aeration and/or underwater jets. (Click Image to Expand)


While the question is easy to answer, creating such an environment is what makes the job of water gardeners challenging. There are many steps to this process, and one of the most basic is understanding what can cause toxic conditions that degrade the quality of the water— and consequently, fish health.

One of the most common contributors to harmful conditions is what I call a “dead zone.” This is an area in the pond that has very low oxygen (O2) levels—where aquatic life either dies, or leaves if it can. I have referred to this topic in some of my previous articles (e.g. “A Hobby Not a Chore” in POND Trade July/August 2014), but in this article I want to expand on all aspects of this very important topic.

Dead Zones: Where, How and Why

A dead zone can develop in a small, quiet area in the pond that has a low circulation rate, or it can occupy the entire pond. This zone can be completely devoid of oxygen, but in most situations the oxygen level is just low. Fish and most aquatic life need around 8 mg/l of oxygen to be healthy. Fish can survive with slightly lower levels, but they will be stressed just because they have difficulty breathing. Stressed fish mean that diseases and parasites can become a problem. Lower oxygen can also kill off your ecosystem and the critters (bacteria/invertebrates, et cetera) that keep ammonia and nitrite levels low. Higher-than-normal levels of ammonia and nitrite can cause toxic conditions that will put the fish in serious jeopardy. This entire situation is not acceptable for either the fish or the ecosystem.

Toxic Algae and Decay

When the ecosystem starts dying it will be evident in the degraded clarity of the water as well as certain algae becoming more prevalent. The single-celled planktonic algae that cause pea-green water can proliferate (more on this later). Low oxygen can also cause a white/grayish sheen to appear, not only in the water, but also in the biofilm (the patina of the pond). This white/gray color in the biofilm is made up of the dead critters that were once alive in the patina. The white/gray color in the water itself is the dead bacteria and invertebrates that are still suspended in the water. The fish may still be alive, but if conditions are left to deteriorate further…well, enough said. I have not talked about this white/gray color before in my writings because it does not occur all the time. However, I usually see this once or twice a year in clients’ ponds.

 Other very toxic algae that can appear in our ponds are photosynthetic bacteria called blue-green algae. Another name for these algae is Cyanobacteria. It is most often blue-green in color, but can also be all-blue or all-green, and also reddish-purple or brown. This type of algae is usually mat-forming, shiny and slick feeling…but not always. It grows in our ponds in dead zones or where there is slow-moving water that has a lot of excess nutrients like nitrogen and phosphorus. This kind of algae is very toxic to life. If it is present, take measures to reduce the inflow of nutrients and increase circulation to eliminate the dead zone. I normally only see this in ponds that have dead zones.

Other evidence of dead zones can be smelly water, such as the smell of “rotten eggs,” which is an indication of low oxygen. Water should smell “of the earth”—like healthy dirt, as I like to say. It should not be an offensive odor.

A buildup of organic matter and sediment in a pond can also create a dead zone by depleting the oxygen as it decays. These areas can be highly toxic, especially during the winter months when the cold temperature slows the fishes’ metabolism. Fish enter these zones and cannot swim out. I view these areas like humans view confined spaces, especially underground. Toxic gases can accumulate in these areas, and when we enter them we can succumb so quickly that we cannot get out fast enough. Fish can enter a dead zone area in the pond during a time when they can barely “flick a fin,” and before they realize the problem, they die. So removal of excess organic matter is essential—especially before the pond freezes.

Detective Work

So what else should we look for in the detective work of determining whether a pond has dead zones? Certainly pea-green soup water is obvious, but this is not a positive indication of dead zone areas. This condition can be caused simply by the “new pond syndrome” or “newly power-washed pond syndrome.” Those are when the natural biological systems in the pond have not been established yet, and certain algae take advantage of that. Adding beneficial pond bacteria can help speed up this colonization process and restore balance. Using ultraviolet light filters will also eliminate these algae if the filters are maintained. However, UV can mask the evidence of dead zones. An established pond that has been up and running without a UV filter and has the pea-green soup condition is certainly a candidate for the dead zone problem.

Other evidence of dead zones includes fish gasping for air. This seems obvious, but if not looked for at the right time this sign can be missed. Early morning is the time when oxygen is the lowest after an entire night of the fish, the natural algae in the patina and plants in the pond using oxygen. This process in plants is reversed during the day when photosynthesis actually produces oxygen. So look for gasping fish, especially in the early morning. The early detective work of solving the problem may save your fish and ecosystem.

Dead Zone Pond

The main pond body is shown at left, while the section of stream that’s shut down during the winter is at right. Fish could have easily swum up the relatively shallow channel of the stream and entered the dead zone. They can quickly die in this area before they can get out.

Dead zones can easily be created when pumps are shut off for the winter. This is when aeration needs to be added in these potential quiet areas.

I like aeration, as opposed to water pumps, to create water movement at this time of the year for a couple of reasons. First, the intake of the water pump can plug during the winter and is difficult to unplug when the pond is iced up. Also, these water movement areas, whether created by a water pump or aeration, can dome over with ice. When this occurs with a water pump, the water is sealed off from exchange of oxygen and toxic gas with the outside air.

However, with an air pump you’re adding oxygen under the ice and the gases are still driven off.

Next Steps

OK, through our detective work we have determined that the potential exists for dead zones, or we have seen the actual evidence. What are the next steps?

Fencing was installed along the bridge to prevent fish from swimming up the shut-down stream/channel.


Well, I know that this seems after-the-fact, but it bears mentioning that the design of the pond should have taken the potential for dead zones into account to begin with. Over-complicated designs that include islands and deep pockets can contribute to the potential for dead zones. So, if the pond has these types of areas, be aware of this and monitor them closely.

Adding circulation is a must to eliminate dead zones. We cannot depend on natural wind circulation like that in a lake in our water gardens due to the small size of our ponds, and also because we have them in protected situations. We can add a bigger water or air pump, perhaps. Adding underwater jets in the areas that seem stagnant is relatively easy. Most of the time, I like adding aeration to still or quiet areas to reduce even the potential for a dead zone.

Adding plants is a huge dead zone eliminator. Plants can help keep the water fresh by utilizing fish waste and dissolved organics for growth. Plants are a natural filter that take up the waste as well as adding oxygen to the water.

Spring startup of pumps can be a hazard for the ecosystem when they are first turned on. Stagnant water that collected in a “pit” (a deep spot in a stream during the winter), or an area that did not have any kind of circulation, can be a highly toxic dead zone. When the pump is started this toxic water would return to the main pond. The water in these areas should be pumped out separately and discarded before any of it returns to the main pond.

A New, Oxygenated Age

The stream is running and the fish are happy in the zone that was potentially deadly to them during the winter.

deadly to them during the winter. (Click Image to Expand)

Many years ago—perhaps 20 or so—many ponds had dead zones. Today, with improved water pump efficiency and a variety of water and air pumps available, the ponds are so much better circulated. Also, we are getting smarter and understanding our ecosystems better so that the mistake of having dead zones is less of a possibility.

So, do the detective work to determine if dead zones exist. Look at and smell the water. Gasping fish, especially in early morning, are a sure sign of low oxygen levels. Also, look for areas in the pond that are not getting much circulation—where the water is very calm. In these areas you could have a proliferation of blue-green algae, given the right nutrients. If you see a whitish cast to the patina or to the water itself, then there is a problem with low oxygen. The solution to a dead zone, or an entire pond that has a problem, is to simply add circulation and/or plants.

2 Responses to Are Dead Zones Killing Your Fish?

  1. Glen Gordon August 20, 2015 at 11:37 PM #

    Hi, I have written to you in the past after implementing your grid veggie filter. I read your article Are Dead Zones Killing Your Fish. Although this is not a problem for my pond I do have a lot of what I think is sediment and organic material building up in my stream bed. The stream bed is two tiered with upper level being fed by the grid veg filter (see attached pic). I included a pic from the lower stream showing the “gunk” that has built up. Twice a week I remove string algae from both stream beds and some of the gunk floats into the pond. The fish come and seem to feed off this stuff every time I remove the string algae. The edges of the stream are lined with various plants which I assume is contributing to the gunk. Do you think I need to periodically clean the gunk from the stream bed? I did this once in June with a wet vac but was concerned I maybe pulling good stuff out of the water. The pond is doing great. Trying to stay ahead of any potential problems.

    Your input would be greatly appreciated.

  2. Jamie Beyer August 20, 2015 at 11:38 PM #

    Good question Glenn.

    First of all when the detritus is stirred and the fish can get to it — they are after the small critters (micro organisms — invertebrates) to eat. This is their natural foods. It looks to me from the pics that this layer of sludge is not too thick. This is good. It is actually healthy for a pond. Where the detritus builds up so much that when stirred it smells like rotten eggs then this is not good. This layer has become anerobic (without oxygen) and can be toxic to life forms that need oxygen. In the stream it is not so bad due the fish not being able to get to it. In the pond they can stir it and then the toxins can be released. So, you should be okay even if it builds up more. However, get rid of it this fall or next spring.

    Good luck and have fun water gardening.

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