Biological Pond Cleaning Through a Filter Trench

Biological Pond Cleaning Through a Filter Trench

In your garden pond, clean water is essential to the fish, and other animals feel comfortable in it. You can ensure clean water with filter technology. In addition, you need to clean your pond at least once a year to remove silt from the bottom of the pond. The mud is stirred up by the fish and can promote the formation of algae. If you want to avoid expensive technology and a lot of effort in clarifying the water, you can create a filter trench.

Why a Filter Trench Makes Sense

Even if you have a lot of water plants in your garden pond and clean the pond twice a year, deposits will form on the bottom of the pond over time. Parts of the plants die, fish excrements and uneaten fish food, leaves, pollen, and other impurities sink to the bottom. The fish feeding at the bottom stirs up this sludge. Nutrients enter the water, and it becomes turbid. This turbidity favors the formation of algae.

If you do not want to invest in expensive filter technology, largely avoid algae removers and reduce the effort for cleaning, you should create a filter trench.

Filter Trench Suitable Only for Larger Ponds

The filter trench is a natural filter chamber. However, it is only suitable for larger ponds such as fish ponds and swimming ponds. Depending on the pond function, the filter trench must take up a percentage of the pond’s volume. For ponds with a lot of water movement, for example, swimming ponds, the filter trench must be enormous. The filter trench should account for at least 20 percent of the pond area in these ponds. If you have a fish pond with a high fish population, it is even recommended that the filter trench be as large as the pond itself.

Functioning of the Filter Trench

The filter trench is located near the pond and is connected to the pond via an underground hose. Similar to a filter pump system, the filter trench constantly flushes the pond water. The cleaned water returns to the pond. In most cases, you cannot do without a circulation pump at the end of the trench run, even with a filter trench. In a filter trench, the water is not cleaned by filter inserts but by plants.

Advantages of the Filter Trench

A filter trench has a natural look and can flawlessly integrate with the surroundings and the pond environment. It has a natural function. The plants provide clean water, but they also remove pollutants such as nitrates from the water. The filter trench provides a biological balance in your garden pond.

How the Filter Trench Should Be

The filter trench should be as straight as possible. This will also make it easier to build and line with foil. For larger ponds, the filter trench should be connected to the pond by several hoses. The connections are suction points that have a suction pipe and suck the sludge into the filter trench.

There is a collection box where the suction pipes end in the filter trench or the dam between the pond and the filter trench. You should provide a separate gate valve for each suction pipe. The suction power of each pipe can be adjusted separately. The pond size and the flow velocity determine the size and length of the filter trench. For suspended solids from the water to settle in the trench, the flow through the trench must be slow. This ensures that they do not end up back in the pond at the end of the section.

It would help if you filled in the ditch in the initial area with lime-free pebbles. It is best to place the filter trench on a long edge side of the garden pond. In this way, it takes up little space and hardly disturbs.

Pond and ditch water should be separated from each other over the entire run. You can create a dam with soil material from the ditch excavation. However, you can also use a partition wall. With stone slabs or wooden planks, you can visually disguise the dam or partition.

Pond Pump at the end of the Filter Trench

At the end of the filter trench, you should install a pond pump. Since it sucks the water from the filter trench, water from the pond can continuously flow into the filter trench due to the gradient. Coarse particles and suspended matter can be deposited in the filter trench. This prevents the pond pump from clogging and keeps it running maintenance-free for a long time. Particles that are still in suspension are filtered out. You can couple the pond pump with a filter system to return the water to the pond. For the sediment to enter the filter trench, it must be stirred up in the pond. In a natural pond with plants and no fish in it, this does not work as efficiently. If fish are in the pond, they can stir up the sediments when they feed on the bottom.

Plants for Cleaning

Cleaning is done in the filter trench by plants. Plants can absorb nutrients that are dissolved in the water and also absorb carbon dioxide and produce oxygen. The plants counteract the formation of algae and prevent the growth of algae. The aquatic plants break down suspended solids and amino acids. They convert harmful nitrate to nitrogen.

Good clarifying properties have

  • Calamus
  • Frogbit
  • Water star
  • Cattail
  • Crab claw

Besides these clarifying plants, you can also use plants in the filter trench with a high decorative value.

Besides the plants in the filter trench, microorganisms also ensure the cleanliness of the water. They extract nutrients from the water as well as phosphate and convert amino acids into ammonium.

Arrangement of Plants in the Filter Trench

It depends not only on the choice of plants but also on their arrangement in the filter trench. The clarification course should be designed from coarse to fine. At the beginning of the filter trench, you should plant floating leaf plants and reeds. For the back of the filter trench, finer branched underwater plants are suitable. Fine-leaved plants trap the coarsest suspended particles. The reed plants take up particles of medium size. Densely branched aquatic plants can also pick up fine particles.

Plants for the Different Areas of the Filter Trench

The filter trench should have underwater plants as well as shallow and deep water plants.

Submerged plants are good oxygen producers. They are entirely covered with water and produce oxygen mainly through the root or rhizome. Waterweed is one of the best filter plants and is a typical underwater plant. Waterweed is finely branched.

Good oxygen producers are also watermilfoil, hornwort, and water hose plant. These free-floating plants provide shelter for many small creatures. With their fine branching, they trap many suspended particles.

Suitable shallow and deep water plants include pike arrowhead, water mint, frog spoon, bulrush, and water lilies. It would be best to choose smaller species for water lilies, as they can quickly grow to a considerable size. You should make sure that the floating leaf plants still allow enough light to reach the underwater plants.

Control the Growth of the Plants

Aquatic plants are essential for good water quality and are the heart of the filter trench. However, you should make sure that there are not too many plants in the filter trench. Therefore, you should control the growth of the plants. You should divide plants that have grown too large.

Reed plants can provide an excellent filtering effect, but you should use reeds sparingly. It can multiply quickly and overgrow the filter trench. The filter trench can then no longer fully perform its function.

While species richness is good, you should not get bogged down with species. How many plant species you use depends on the size of your pond and filter trench. Even for large ponds and filter trenches, you should use five different plant species for underwater plants, floating plants, and plants for the shallow water zone.


A filter trench is suitable for larger ponds and can clarify the water. The clarification function is performed by plants, while you can primarily do without technology. However, it would be best if you integrated a pond pump into the filter trench. The filter trench is separated from the pond with a dam or a partition wall. The water enters the filter trench through hoses or pipes. For the filter trench to clarify the water, it is crucial to choose the right plants.

Florian Egert

I am Florian Egert, the owner of I live with my wife and two children in Germany in a small village in the countryside.

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