Bingo - there you go
It takes regular cotton floss - line the whole filter with it, surrounding the green base of the pump. There should be a green platform with many holes in it (like an undergravel filter board) on the bottom of the "box" (I just saw that you do have it - but yours is white.)
It takes regular cotton floss - line the whole filter with it, surrounding the green base of the pump. There should be a green platform with many holes in it (like an undergravel filter board) on the bottom of the "box" (I just saw that you do have it - but yours is white.)