A new filter for a five gallon tank?

[SciFyDi]

Hi,
I was just wondering if anyone can recommend a filter for my Betta who lives in a five gallon tank. I currently have the API size 10. It seems to have stopped working as I have noticed that after a week the filter is covered in gunk. I usually could go two to three weeks without changing it.
Thanks

 

[cgthebeast]

I prefer spong filters, easy to clean and less current for the betta

 

[Autumnsky]

Aquaclear model 20 it is adjustable for flow.