Aeration

• Bubbles agitating the surface, promoting gas exchange (aeration).bubbles in aquarium image by sasha from Fotolia.com
  
  The most important purpose of an air pump is to aerate the water, increasing dissolved oxygen so fish can breathe. The pump does not force oxygen directly into the water, but rather agitates the surface so oxygen is dissolved more quickly.
  
  

Circulation

• The column of bubbles rising from the end of the airline tubing forces water to move with it, especially if the column is contained within a tube. The movement keeps the water from becoming stagnant.
  
  

Filter Operation

• Many kinds of filters are powered by air pumps. These include corner box filters, sponge filters and many undergravel filters.
  
  

Decorations

• Many fish tank decorations are designed with moving parts powered by an air pump.
  
  

Sick Fish

• Sick fish should be isolated from the main aquarium in their own "hospital tank." In an emergency, an air pump can keep a fish alive in an unfiltered tank for at least a day.