The choice of air pump depends on the purpose you are going to use the pump for. Generally air pumps are used for: 1) Ornamental Purpose like turning valves, spinning wheels, lifting things, creating artificial underwater waterfall etc. 2) Air stones in the tank which generate bubbles or give off a fine mist of air. 3) Some filters, such as corner filters and under gravel filters are driven by air and would require an air pump. 4) Some underwater habitats for semi-aquatic animals, such as Newts, Crabs, Mudskippers, and some Frogs or Shrimp also require an air pump to keep fresh air circulating into the enclosure so that these animals do not crawl out into an unaerated environment. 5) Air pumps can also be used to create a current in the water, to prevent parts of the tank from becoming stagnant. 6) Air pumps are required for some types of protein skimmers to operate in marine tanks. Two of the key features of any kind of air pump is to increase the surf...