Pond aeration - Help, my pond is suffocating!

Pond aeration - Help, my pond is suffocating!

Have you ever worried about whether your bacteria at the bottom of the pond are getting enough oxygen? No? Then you are in good company! We pond owners usually only come into contact with the subject of oxygen when the fish are panting at the water surface in midsummer or when something goes wrong in winter and fish die. And yet oxygen is actually the most important factor in your pond! Take a few minutes to become an expert on the subject of oxygen. 

Let's start with something simple: If you put a glass of water on the table, the oxygen from the air (21% O2 content) will dissolve in the water. The maximum oxygen concentration is temperature-dependent. At 20°C, 9.1 mg/l oxygen can dissolve in your glass of water or pond. At higher temperatures the value decreases and at lower temperatures it increases. That’s why it’s very important in winter not to disturb the lower water layers in your pond, as this is where the lowest temperature and the highest oxygen content are to be found. For this very reason NEVER position your pumps or aeration at the lowest point in the pond - about 60 cm above the lowest point is better. Just as if you stir your glass vigorously, the oxygen content will increase, because the movement increases the surface area like the waves on the sea or lake, so in your pond, every water spout, waterfall or stream increases the water surface of your pond and thus increases the oxygen content.  

There are a few situations where additional pond aeration is not just a nice-to-have, it’s a must!

1. High summer

Due to the rising water temperatures, the oxygen content in the water decreases. If your pond is deeper than 100 cm, the cooler water will be at the bottom and so there will also be a higher oxygen content there. Again, please do NOT place your aeration at the deepest point, as this will mix all the water layers together.

2. Winter

In winter, the highest oxygen content is again measured in the deepest water layer. Since water has the highest density at 4 °C and is therefore at its heaviest, this is where your fish will stay. Other pond inhabitants, such as frogs, also spend most of the winter in the mud at the bottom of the pond and only very rarely come up for air. However, if your pond freezes over completely, you may experience oxygen problems. The smaller and shallower your pond, the more likely it is to have oxygen problems. A remedy is quite simple: make or buy an ice-free holder (polystyrene bonnet) and buy a small air pump (such as  JBL PondOxi-Set ) that releases air into your pond water through a hose via an air stone. The rising air bubbles will immediately aerate your pond water and also move your pond surface, preventing it from freezing over, even in very severe winters.

Aeration prevents freezing

3. Algae bloom

If you are one of the stricken pond owners whose water is completely green (floating algae) or if you can hardly see the bottom of your pond due to filamentous algae growth, there is the following danger: During the day, all algae happily produce oxygen. At night, however, they consume it, albeit often to a lesser extent than produced during the day. Strong algae growth can lead to the oxygen content being consumed so much that the fish can have difficulty breathing towards the end of the night.

Filamentous algae in the pond
Floating algae in the pond

4. Algae control

If you have used an anti-algae agent, the dying algae will be broken down bacterially unless you remove it from the pond. The decomposition process devours vast amounts of oxygen and is very likely to lead to oxygen problems in your pond. Therefore, additional aeration is STRONGLY recommended in connection with algae control. 

5. Feeding

At a water temperature of 10°C, your fish normally need 45 mg/l oxygen per kilo of fish per hour (a 30 cm koi weighs about 470 g, a 60 cm koi about 3.8 kg). At 20°C the oxygen requirement doubles to 90 mg/l per kg/h. However, it is important to know that in high summer temperatures your fish need 500 mg/l per kg/h when feeding (even at 20°C)!!! This is exactly when the high temperatures result in the pond having its lowest oxygen levels. 

6. Sludge decomposition

Few ponds contain little or no sludge at the bottom of the pond and unfortunately leaves and dying plants only contribute to this. The bacteria living at the bottom of the pond try to break down your pond sludge, but often don’t have enough oxygen to work effectively. The higher the oxygen content of your pond water, the better the bacterial decomposition processes at the pond bottom, even if your pond aeration doesn’t reach the bottom.

O2 deficiency at the bottom leads to putrefaction processes in the pond
Decay processes due to O2 deficiency

7. Koi ponds

Koi ponds are usually designed differently to normal ponds. They normally don’t have shallow water zones and their pond walls are often steep. As a result, the water surface is quite small in relation to the water volume. The natural oxygenation will definitely NOT be sufficient.

Get an idea of the oxygen content of your pond water with the help of an oxygen test (e.g.  JBL PROAQUATEST O2 Oxygen ). Remember not only to take pond water from the surface, but try to get a water sample from the bottom of the pond (not easy but fun!). O2 values below 3-4 mg/l can be critical for your fish. If your pond does not have any additional water movement from a water spout, stream or small waterfall, you’ll need to create an additional pond aeration system. It is a small investment with an extremely beneficial effect!

Tip: Clever little tool: The  JBL PondOxi-Set contains everything you need for your pond aeration in summer: air pump, 10 m air hose, bubble stone and - very helpful - a float to help you position the bubble stone at any depth you want. The float is also available separately under the name  JBL Floater + AntiKink .

JBL PondOxi set
© 07.11.2021
Heiko Blessin
Heiko Blessin

Tauchen, Fotografie, Aquaristik, Haie, Motorrad


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