Koi lovers are well aware of the importance of monitoring pollutants such as ammonia and nitrite in their koi ponds. However, one vital factor that is often overlooked is the oxygen levels in the water. Oxygen is an essential gas that is easily dissolved in cold water, but as water temperature increases, it becomes more difficult to dissolve. This can have serious consequences for fish, especially in garden ponds during the summer months.

Fish require oxygen to survive, and they must extract it from the water they live in. When dissolved oxygen levels are low, fish can experience oxygen debt, which means the amount of oxygen used by fish to breathe is greater than the amount of oxygen found in the water. If the dissolved oxygen levels are not quickly increased, fish can become stressed, prone to secondary diseases and infections, and even die.

What Affects Oxygen Levels in Koi Fish Ponds?

The number, size, and species of fish in the pond have an immediate impact on dissolved oxygen levels. Larger fish consume more oxygen than smaller ones, and some fish species require more oxygen than others. Typically, low dissolved oxygen levels are first noticed when the largest fish appear to die for no apparent reason.

Aquatic vegetation also has a significant impact on dissolved oxygen levels. During the day, all plants produce oxygen as a waste product in a process called photosynthesis. At night, however, photosynthesis stops, and aquatic plants actively respire, using the nutrients they created during the day. This can lead to decreased dissolved oxygen levels at night, especially in garden ponds during the summer months when oxygen is relatively difficult to dissolve in water. To counter this, it can be helpful to regularly trim underwater species and provide oxygen, especially during the summer, when growth is vigorous.

It's important to recognise that aquatic plants play a crucial role in removing the ammonia waste created by fish and maintaining a balanced water environment in captivity. In nature, the number of aquatic plants is constantly reduced by fish, ducks, and geese, helping to maintain this balance.

Bacteria and other microorganisms in the filtration system also require oxygen to breathe. The chemical conversion process of ammonia to nitrate uses oxygen, and bacteria are very efficient at taking oxygen from water, which means they can take as much as they need, leaving the rest for the fish. Therefore, it's crucial to ensure that the filtration system is fully aerated, as this can increase the ability to provide oxygen for the fish.

How to Monitor Oxygen Levels in Your Koi Pond

Gasping at the surface of the water is a last resort for fish to get oxygen. The first sign of low dissolved oxygen is when your koi become sluggish and stop eating. As oxygen levels decrease, fish become less energetic, and feeding will use up more oxygen. Usually, this is when you'll notice that something is wrong, but adding medication to the water can actually reduce oxygen levels even further, leading to fatalities. This can result in the misdiagnosis of a fish disease.

To monitor oxygen levels in your pond, there are testing kits available. The recommended minimum dissolved oxygen requirement for koi is about 6 mg per litre. It's important to note that this is the minimum requirement for healthy growth, tissue repair, and reproduction of koi.

Conclusion

Monitoring oxygen levels in your koi pond is essential to the health and wellbeing of your fish. Regularly checking and maintaining dissolved oxygen levels will help to prevent stress-related diseases, infections, and fatalities, and will allow you to create a balanced water environment that your fish can thrive in.