This is a good quick easy way to give your small tank a little boost in bio filtration on the cheap. There is nothing fancy about it and its kinda ugly, but it works if you have nothing else handy or you're broke. 
You take a large sponge, and cut out the center to form a box. Leave a bottom, you aint making a TUBE.
This example is about 5x3x5 inches but it's only limited to how big you want the sponge.
With some wire, secure the sponge box so the water from your undersized-for-the-job hang on the back filter flows right into the "box". the water will seep out almost as fast as it goes in.
Since most of these HOB filters have a bio pad in them, they shed bacteria right into the sponge box, and you quickly get a nice colony of bacteria.
When the water begins to overflow the sponge box, you just unclip it, squeeze it out and put it back.
I suppose you could leave it there and use it as a permanent bio filter in a small tank, if you can find a sponge that has an esthetically pleasing color.
But a yellow one, they turn brown in a few weeks.
If the sponge cells are small and tight together, it has the added benefit of cleaning your water a second time without having to add another HOB filter to do it.
It's also handy for clearing up dirty water when you've been poking around in the tank, since the tiny stuff that goes right through fiber filter pad wont go through a good dense sponge.
I know, not all my ideas are good ones. I do try.
You take a large sponge, and cut out the center to form a box. Leave a bottom, you aint making a TUBE.
This example is about 5x3x5 inches but it's only limited to how big you want the sponge.
With some wire, secure the sponge box so the water from your undersized-for-the-job hang on the back filter flows right into the "box". the water will seep out almost as fast as it goes in.
Since most of these HOB filters have a bio pad in them, they shed bacteria right into the sponge box, and you quickly get a nice colony of bacteria.
When the water begins to overflow the sponge box, you just unclip it, squeeze it out and put it back.
I suppose you could leave it there and use it as a permanent bio filter in a small tank, if you can find a sponge that has an esthetically pleasing color.
But a yellow one, they turn brown in a few weeks.
If the sponge cells are small and tight together, it has the added benefit of cleaning your water a second time without having to add another HOB filter to do it.
It's also handy for clearing up dirty water when you've been poking around in the tank, since the tiny stuff that goes right through fiber filter pad wont go through a good dense sponge.
I know, not all my ideas are good ones. I do try. 
