How to Build an Aquarium Filter

• 1). Measure the circumference of the plastic cup with a circumference compass. This measurement is what you will use to draw a circle on the foam.
  
  
• 2). Using the compass, draw a circle on the egg carton foam. Try and get it as close as possible to the actual measurement. It is better to go a little bigger if needed, because you can always trim around the circle and make it smaller.
  
  
• 3). Cut out the circle from the foam. Test to see if it fits in the plastic cup. It should touch all sides of the cup to properly filter the fish tank water.
  
  
• 4). Attach the pump tube to the inside side of the filter, using hot glue. After if is completely dry, do a test run. If the filter is too thick and backs up the water, thin the foam circle by cutting it about 25%. If this is still too thick, continue to trim until the filter works adequately.
  
  
• 5). Take out the filter and hand wash or replace once a week. It should last at least a month, and probably longer if you rinse it regularly. Do not use soap or dish detergent to clean the filter. Use fresh, running water.