Many people tie aeration and temperature together by pointing out that the warmer the water the less oxygen it contains. This is true but is not that significant. 86-degree (30° Celsius) water contains 10% less oxygen than 75-degree (24° Celsius) water. So why do fish “pant” at the surface when the temperature is raised?
This has to do with fish metabolism. Fish are cold-blooded. The metabolism of cold-blooded creatures is linked to the temperature. The higher the temperature the greater the metabolism rate. For fish this increase is significant. So, fish need more oxygen at higher temperatures, considerably more oxygen.
If fish are subjected to high water temperatures, such as 100 degrees (38° Celsius), they die rapidly. What people miss is that they don’t die because the water somehow burns them. They die because their metabolism speeds up and their gills can’t oxygenate their bodies fast enough. Water contains much less oxygen than air, so gills don’t function as well as lungs.
Note that a warmer aquarium like a discus breeding aquarium has about 10% less oxygen in the water and the fish will have a higher metabolism. For this reason, even with high aeration, warm water aquariums need to have fewer fish in them than colder water aquariums. Temperature and metabolism are also linked to lifetime. The higher the temperature the shorter the lifetime of a fish.
Source: Aquariumscience.org – David Bogert