Fish need oxygen just like land animals do. Fish absorb oxygen directly from the water into their bloodstream using their gills. Fish cannot live without oxygen.
Oxygen depletion is the cause of many fish kills.
Low oxygen stress is common causes of disease outbreaks. The amount of oxygen in water decreases as temperature and altitude increases. As temperatures increase, fish metabolism will increase so they consume more oxygen. So both the increase in temperature and increase in fish metabolism may cause oxygen depletion in the summer. Since carp are typically stocked at high densities, there is a greater demand for oxygen than in a pond or lake that you would find in a natural environment.