It's very easy to make your own sponge filter. The small filter sponges that comes with most filters are not large enough for big tanks so it's advisable to make your own.
This is not a very good looking filter but it's very powerful and you wont need to clean it more than once every six months if that.
To make it you need a sharp kitchen knife, a peice of hose, a hammer, a nail, a plastic tube like the ones used for electrical installations, a peice of filter sponge and a powerhead.
The powerhead should be of suitble power for your tank. This means that it should pump at least twice as much water per hour as your tank contains.
Filtersponge can be bought at most pet-shops. I have found it better to use sponge with larger pors since it seems to house the beneficial bacteria better without clogging up.
In this example I have bought a big cube of filtersponge to make two large triagular filters for two 55 gallon tanks in my fish room. Since this will be breeding tanks in a fishroom I make large filters and it does not matter that the sponge is blue. I have seen black, green and blue filtersponge on the market but after a while they all look about the same so it is not all that important unless you need it to look natural from the very start.
I cut the cube diagonally into two triangle shaped peices.
Mark out how much of the plastic tube that will be inside the sponge.
Then make holes with a hammer and a nail all over the part of the tube that will be covered by the sponge. Drive the nail all the way through so that you make two wholes at a time.
The wholes can of course also be made with an electric drill.
You need to plug the bottom end of the tube with something. I have used a peice of styrofoam in this example.
First make a small hole in the sponge with your finger and then you can push the actual tube all the way to the bottom of the triangular peice of sponge.
Then you place the powerhead on top of the tube and conect a hose to the outlet so that you can point the out going water flow at the surface in order to get more oxigen into your tank water.
Here is one of my filters when it is mounted in a tank.
Easy to make, cheap and powerful....but pretty ugly. Good luck with your filters.