Here are some guidelines to keep in mind when you are choosing an air pump for your aquarium;

• The more things you are running from your air pump, the larger air pump you will need, and equally, the fewer things you are running from your air pump, the smaller air pump you will need.
• The more resistance the things you are pushing air through have to the air, the larger an air pump you will need. (Larger air stones will provide more resistance than smaller air stones, and any air stone will provide more resistance than no air stone. Also, some kinds of air stones will provide even more resistance than others of the same size.)
• When choosing an air pump, pick one a little larger than you initially think you will need. This larger air pump will help you by compensating for underestimating when choosing the air pump or by providing a little room for expansion if you decide you want that later.