I don't think there is nessersarily anything wrong with carbon. But I have only used it to strip meds etc from qt tanks.
My understanding (and it could be wrong) it that carbon filters sponges/stick type things, will absorb so much that at some point they can't handle anymore and spit it all back in your tank. That's why the manufactures of carbon media tell you to replace on a regula basis. And always after med removal.
Now this could be really really wrong, but that's why I have always changed it out. (and you sometimes find that because a filter is little they use carbon more)
As for other sponge or media, you can jig also lots of things into all sorts of spaces. Cut up large pond filter sponge, filter wool, pours us beads filter noodles, washing up sponge, nylon scourer thingies. A mixture of a couple of the above,
You want to use a sponge of some sort in the top of the media holder bit though if I remember right, so you don't have pokey bits getting dragged into the motor..
Used your imagination (also pinch ideas from you tube) and by being flexible and willing to use scissors you can save a fortune.
