Air pumps are used to ensure that the water in a fish tank is constantly moving. They are needed when there is a low amount of oxygen in the water. A 5-gallon fish tank needs an air pump with a power rating of at least 15 watts. A fish tank with an air pump can be very beneficial for your pet's health and happiness. There are many types of air pumps out there, but they all have the same function: to circulate oxygen in the water and keep it from becoming stagnant.