Oxygen is as important to fishes as it is to us for survival. We can't sustain even a minute without oxygen. However, fishes in the aquarium can survive longer than us when there is depletion of oxygen content in the aquarium.
When there is a power outage, the machine that circulates the water stops and so does the heater. The main purpose of water circulation is to distribute oxygen equally throughout the aquarium. When the water at the surface is rich with oxygen, the machine brings it down and the cycle goes on.
When the machine stops, there is no circulation of oxygen. The fishes consume oxygen available at the lower levels of the aquarium. Then it starts hitting the surface of the water for more oxygen. Nevertheless, the fishes are under a lot of stress and they can only survive for almost two days after the oxygen depletion.
The main purpose of having filters in your aquarium is to filter out anything that contaminates the water and harms the fishes. There are three types of filters. Mechanical, chemical, and biological filters.
When water passes through a mechanical filter, it removes large particles that get stuck in the filter because it cannot pass through.
Chemical filters are chemical resins or carbon. They filters out any toxic chemicals present in the tank.
Biological filters are bacterias. These break the toxic chemicals into fish-friendly chemicals.
Filters not only cleans the aquarium, but it also removes toxic elements, makes the water breathable for the fishes, and makes the fish tank a comfortable home for fishes.
When there is a depletion in oxygen, the bacterias won't survive. The ammonia and nitrite excreted by fishes and produced during decomposition at the bottom of the aquarium, makes the water more and more toxic for the fish. Eventually, it causes the death of fish. The whole process can take up to 12 hours.