The Practical Guide on Pond Sludge

You’ve Got a Sludge Problem. Here’s Why and What You Can Do About It

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-Written by Aquatic Biologist and Outsides Sales Member, Liz Edgerton

If you have a pond, you’ve probably noticed a greenish-black mucky layer on the pond bottom. If your pond is young, it may not be much of an issue but as the pond ages, you’ll begin to notice the layer getting thicker, covering the bottom until you can no longer see the dirt, rock, or sand that was once visible. You may also notice a foul, rotten egg type odor when the pond turns over in the spring and fall, or when swimming or other activities disturb the bottom. You’ve got a sludge problem. Why is this happening? And what can be done about it?

What is Pond Sludge?

Pond sludge is a buildup of organic materials that accumulate in your pond over time, mixed with inorganic materials like sand, clay, or silt. The organic material includes animal waste, uneaten fish food, leaves and grass, dead algae, fertilizer, grease and oil from runoff, etc. As these organic materials settle in your pond, they begin the decomposition process. Decomposition requires oxygen, which is pulled from the water. While small amounts of organic material won’t cause an issue, larger accumulations can greatly reduce the amount of oxygen in the bottom of the pond, resulting in an anoxic layer of water and sludge in the bottom of your pond. In the anoxic layer, decomposition cannot take place, and the sludge layer will continue to increase as more organics end up in the pond. It’s a vicious cycle. This anoxic layer of sludge also plays host to unwanted anaerobic bacteria that produce hydrogen sulfide. Hydrogen sulfide is the culprit for the rotten egg smell that you notice when the pond bottom gets stirred up.

Sludge buildup is part of the natural process of any pond. If left to their own devices, ponds will go from healthy and clean to stagnant and eutrophic, slowly filling in and turning into wetlands, and then eventually reverting back to grassland. In pond management, the goal is to maintain a healthy pond and prevent this cycle of succession from occurring.

What Can be Done to Manage Pond Sludge?

Sludge buildup must be managed to maintain a healthy pond, and there are a variety of options available. Reducing the amount of organic material that ends up in your pond is your first line of defense. Be sure not to over fertilize the land around your pond, as excess fertilizer will runoff into your pond and can be a major contributor to unwanted algae blooms and sludge buildup. Trim back and remove dead tree limbs or plants that are in and around your pond, and remove leaves and grass clippings as they enter your pond.

Next, consider aeration. As mentioned previously, the decomposition that is taking place requires plenty of oxygen. Adding either bottom diffused aeration or surface aeration will ensure that your pond has a healthy level of oxygen and will speed the decomposition process along. Kasco’s Robust-Aire Diffused Aeration, AF Series Surface Aerators, or VFX Series Aerating Fountains are all great choices and offer a variety of options based on your ponds needs and your aesthetic preferences. Lastly, be sure your pond has plenty of good bacteria. Just like a human’s digestive system, a pond needs the right kinds of bacteria to help break down the organics in the system. Kasco’s Macro-Zyme Beneficial Bacteria is a mix of pond-loving facultative bacteria that will consume the excess organics in your pond, reducing the sludge layer and rotten egg odor over time.

With the right tools and proper maintenance, you can keep your pond healthy and smelling great. For help in determining the best course of action for your pond, give Kasco a call, at 715-262-4488, or send us an email at

Kasco aeration will help manage and reduce pond sludge.
Pond sludge can be managed with Kasco aeration.
Aeration and maintenance is key to reducing pond muck.
Pond sludge is an issue with older ponds, but can be managed with Kasco aeration.
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