Most pond owners know that cold water can bind more oxygen than warm water. For example, the maximum O2 concentration at 4 °C is 13 mg/l, at 10 °C about 11 mg/l, at 20 °C only 9 mg/l and in hot summers at 30 °C only 7.5 mg/l. That is almost half the oxygen concentration that can be achieved at the greatest density of water at 4 °C.
Now to our subject: We are very lucky that the carp, the father of all koi, is probably the most researched fish of all. This is because the carp has long been one of the most popular edible fish. And people like to research what they are eating - and money is always available for this research! When fish eat, their oxygen demand increases considerably. This is due, on the one hand, to their movement while feeding, because koi only feed calmly when there is no food envy, and, on the other hand, to their food digestion! High-quality and thus correctly composed food ("protein/fat ratio") is easier to digest than food with an unsuitable composition. When they’re not eating, a kilogram of koi consumes about 45 mg/l of oxygen per hour at 10 °C. At 20 °C, the oxygen consumption doubles to 90 mg/l. At temperatures of 30 °C in midsummer it becomes really demanding for the koi. When the koi feeds, the oxygen consumption increases within an hour to, say, 500 mg/l per kilogram of fish at 20 °C! This is also one of the most important reasons why a dedicated summer food is needed when water temperatures rise above 25 °C - and then only sparingly!
By the way, marine water binds about 1-3 mg/l less oxygen than fresh water. Pond lovers who add large amounts of salt to their pond need to bear this in mind, because these amounts sometimes come dangerously close to a brackish water concentration!
Why not take the time to measure the oxygen content of your pond ( JBL PROAQUATEST O2 Oxygen )? It’s best to do this in the morning when the sun is rising. The oxygen content is at its lowest then and you’ll know whether you should increase the oxygen addition in your pond. By the way, below 4-5 mg/l is lethal for the fish...
Find out more about oxygen and temperature in our Themeworld at Oxygen and temperature