Signs of Low Oxygen in Ponds

Fish remain active and, in most cases, eagerly consume food; but this activity requires a properly aerated pond. If there isn’t enough oxygen, the fish may seem sluggish and less eager to eat. They may even hang around entry points of water (like waterfalls, fountains, and streams). But there could be a few reasons why there is a low amount of oxygen.

If you see any of the following signs, your pond will need extra oxygen:

• Fish gasping at the surface of the pond or at any entry points for water (Waterfalls, fountains, streams).
• Foul odours - decaying organic matter (Fish waste, plants, sludge) can indicate that there is a lack of oxygen, water movement or filtration.
• Rapid algae development - This can be a sign of poor aeration and filtration.

Fish gasping for air at the surface of your pond (Useful Blogs: Why do Fish gasp at the surface of the Pond?)

One of the most common signs of low oxygen is fish gasping at the surface of the pond. As above, they may also gather around any entry points of water. It may look like the fish are hungry, but they have activated their survival mode! This should be a cause for concern and it requires immediate attention.

When the pond is seriously low on oxygen, the fish may display odd swimming patterns or they may lay on the bottom of the pond. While this is serious, it is very rare for the oxygen to deplete this much. If the fish start showing these signs, please test your water with a Tetra Pond Test 6-in-1 or a Blagdon Pond Health Test Kit and visually check the fish for any signs of fish diseases.

Foul odours coming from the pond (Useful Blogs: Pond Sludge: How to Prevent and Treat It)

When the oxygen in the pond has been depleted, the healthy bacteria in your pond will stop processing the various chemicals in the pond. This can lead to a build up of sludge at the bottom of the pond and the growth of unhealthy bacteria (or anaerobic bacteria). Both of these combined increases the harmful chemicals in your pond and further depletes the oxygen levels. This also results in foul odours (like a rotten egg smell) emanating from the pond. 

You may also start to see a build up of sludge at the bottom; this is because the healthy bacteria has stopped working and sludge is not decomposing. If you do see a build up of sludge, manually remove as much of it as possible and check for other signs which may indicate a lack of oxygen.

Rapid algae development (Useful Blogs: Green Water and Slimy Algae, Pond Plants)

As above, if the healthy bacteria in your pond has depleted, then harmful chemicals may build up. These chemicals include ammonia and nitrites. Algae, the cause of green water, use these chemicals to multiply. This can cause your pond to turn into pea soup, there may be slimy algae across the surfaces of objects in the pond or you may see a green mat covering the surface of the pond.

If your pond suffers a lot from algae and green water, this should be treated as quickly as possible. The algae will stop sunlight from reaching the bottom of the pond; this reduces the efficiency of your plants and may even kill them. Once more, this has a detrimental impact on the oxygen production of your plants.

As with any change in your pond, you should use a testing kit (either the Tetra Pond Test 6-in-1 or the Blagdon Pond Health Test Kit) to test for a chemical imbalance and treat accordingly.

You may also find the below blogs useful:
• Aeration
• Pond Problems
• Why Temperature plays a big role in Pond Development
• Bacteria: The Importance of Healthy Bacteria

If you have any queries, please email us on info@pondkeeper.co.uk.