There are several ways to oxygenate a fish tank. The simplest way is to use an air pump if you are not using it.
In case of an emergency, you can do the following things to increase oxygen in the fish tank:
1. Pour down the aquarium water from some height into the aquarium
2. Do a large water change (up to 50%) of the tank
3. Use ice cubes

For the permanent solution for low oxygen issue, you can do the following things:
1. Use HOB filter
2. Use a spray bar
3. Use an air pump

If you are facing low oxygen issues during power cuts then you can use a generator-powered air pump to cope with the problem.
In case you want to increase oxygen in your pond using a fountain is a great way to oxygenate pond.

Can you have too much oxygen in a fish tank?Do not add too much oxygen in a fish tank , it is harmful to your fish. It causes lethal gas bubble disease.

Do filters add oxygen in fish tanks?Filters like HOB filters can add oxygen in the fish tank. The filtered water coming out from the HOB filter gets exposed to air before falling into the tank which helps to oxygenate the water.

How can I oxygenate my fish tank without electricity?There are a couple of ways to oxygenate a fish tank without electricity.
The first way is simply pouring down the water of your aquarium using a jar or pitcher from some height it into the aquarium. This way the water will get exposed to the air which will oxygenate it.
The another way to oxygenate the fish tank is by performing a large water change (up to 50%) of the tank water.
The newly introduced water will have more Oxygen, this way you can cope with the low oxygen level problem in your tank.

Can you turn off a fish tank pump at night?If you have an air pump which is separate from the main filtration system of your aquarium then it should not cause any problem if you turn it off during the night.

Why are my fish at the top of the tank gasping for air?When your fish comes on the top gasping for air it is a strong sign that the aquarium does not have a sufficient amount of oxygen.
As the surface of the water has the highest dissolved oxygen fish come to the surface to take oxygen.

How long can a fish live without oxygen?A fish can live up to 2 days without oxygen.

Are bubbles good for a fish tank?Air bubbles help to oxygenate the fish tank. The air bubbles also introduce carbon dioxide in the fish tank that is good for the plants in the aquarium.
The plants take carbon dioxide and produce oxygen for the fish.
Air bubbles also keep the water moving in the fish tank so it reduces the growth of the algae.

Why do fish go to the top of the tank?When there is a lack of oxygen in the tank fish usually go on the surface to take oxygen because the surface has the highest amount of dissolved oxygen.