A sponge pre-filter is beneficial to your tank, as well as your betta, for a many reasons:

  • It catches debris, such as left over fish food or solid fish waste, before it gets inside the filter, where it could accumulate and cause clogging (see our FILTRATION page for more about mechanical filtration); 
  • With it's high surface area, it provides another place that beneficial, nitrifying bacteria can colonize. (See THE NITROGEN CYCLE for a refresher on these bacteria); and
  • The suction created at the filter intake can be enough to catch and snag a betta's long fins as he swims by, leaving them vulnerable to infection. The sponge creates a buffer between the fish and the intake tube, while it  protects their fins from injury.


Sponge pre-filters are quick and easy to make, and are an important part of your betta tank.


  • Large blocks of filter sponge, such as those rated for a tank of 15 gallons or more
  • Scissors


1.  Start out with your block of filter foam

2. Cut out a rectangle of foam that is a bit larger than your filter intake 

  • Only the slotted section of the filter's intake tube needs to be covered by the foam
  • The foam should be longer, wider and taller than the slotted portion of the tube, as pictured in the side view above.

3.  Now you will cut out a channel in the centre of the foam, in which the filter intake tube will sit

  • Cut from one end of the foam almost through to the other end, but not quite! The end of the intake tube needs to be covered as well as the sides.
  • First cut diagonally between the corners, carving out an "X"
  • Next, make four cuts parallel to the sides of your foam rectangle


They to cut a hole in the centre of the sponge that is a bit smaller than the intake tube itself, so that it's a snug fit and will stay on. (the hole outlined in this picture is large enough to illustrate where to cut, but too large for the filter tube!)

4.  Reach into the space that you have cut in the block and pull out the excess bits of foam. 

  • It is alright if the pieces need to be ripped out a bit, just be sure not to pull out too much, or you will create a hole all the way to the outside of the block!
  • Now you have hollowed out the centre of the foam

5.  Place the filter intake tube into the hole that you've created in your sponge.

6.  Trim down the edges and corners of the sponge so that it more closely fits the intake tube (and looks a bit better).

  • It is important to trim the sponge down so that it does not restrict the flow of water too much and cause strain on the filter's motor.


Rinse the foam and the intake tube thoroughly, replace in your filter and you're done!


As noted above, this sponge pre-filter will catch debris that is floating in your tank. If not removed regularly, this debris will rot and contribute to ammonia levels. Your sponge pre-filter should be removed and rinsed in dechlorinated tap water monthly to prevent build-up.