Looks like a filter pad.
You can just go to PetSmart or PetCo, or most other petshops, take this with you, and ask them if they have one just like it.
I'd definitely recommend taking it with you... b/c when I had to get a new BioWheel.... I kept having to go back, b/c it was never the right size.... when I could've easily taken my filter with me... we live and learn.
