This would be similar to a person who has been breathing under a cover for an extended time. Oxygen is used up and the cover must be removed to let in fresh air. In ponds low oxygen usually occurs at night and is lowest just before dawn because phytoplankton have not produced oxygen during the night. Low oxygen can also become a serious problem when Secchi disk readings fall below 20 cm indicating that plankton are too abundant. See details on reading a Secchi disk in Introduction to Fish Pond Fertilization.
Low oxygen can kill fish.
If only a few fish die every day the problem may be disease. If large numbers die suddenly at night low oxygen is probably the cause. Even if fish do not die from low oxygen, they are weakened and more likely to become sick. Suspend fertilizer application until the low-oxygen problem has been corrected and fish stop gulping at the water surface. Add fresh water to the pond immediately to revive the fish, and continue adding water until the fish stop gulping at the surface.