You can buy them cheap on ebay or amazon. Yes, they are powered by an air pump. Simply put, a tube runs through the middle of a sponge and has holes in the tube under the sponge. An air line is inserted down the tube from the top. As the air bubbles up it pushes out water causing it to be sucked in, through the sponge, through the holes in the tube, and out the top. Debris and wastes are trapped by the sponge. Also the sponge acts as a 'home' for benifical bacteria to grow which eats the ammonia as it passes through, giving you clean water out the top. Depending on the amount of fish, such as having one betta in a 3 gallon tank, you might only need to clean the sponge as little as once a month. To do this, take out some tank water, such as when doing a water change (NEVER use tap water as this will kill the bacteria you have waited so long to grow) remove the sponge and put in the container of tank water, squeeze over and over to push out waste, then put the sponge back into the tank.
This picture should help give you the idea....
Last edited by waterdog; 01-18-2013 at 04:13 PM.