What are the signs of oxygen deficiency in fish?

Many of you likely have experience keeping fish. When raising fish, it’s common to purchase an air pump—just in case the fish become oxygen-deprived. So, what signs indicate that fish are suffering from insufficient oxygen?

How do fish react to low oxygen levels?

If dissolved oxygen levels in the aquarium drop significantly, fish will exhibit immediate behavioral changes. First, they become lethargic and less active, often floating near the water surface with little energy or vitality. Their bodies may tilt sideways or even invert, exposing their bellies. Additionally, appetite declines markedly—fish may refuse food entirely, and in severe cases, may attempt to jump out of the tank. Overstocking is one common cause of oxygen depletion, but excessive aquatic plants can also contribute substantially. While many assume aquatic plants always produce oxygen, this only occurs during photosynthesis—which requires light. At night, plants consume oxygen through respiration, competing directly with fish for available oxygen. To mitigate this, supplemental aeration in the evening is recommended.

Oxygen deficiency not only impairs fish activity but also suppresses appetite and may trigger jumping behavior. Several factors can cause hypoxia: overcrowding (requiring reduction in fish numbers), overabundance of aquatic plants (necessitating increased aeration), inadequate filtration, or infrequent water changes.

Regular water changes are essential for maintaining healthy fish. Fresh water contains higher dissolved oxygen levels. Typically, water should be changed on a routine basis: weekly if no filtration system is used, or every two weeks if an effective filter is in place. We hope this information proves helpful!