Take the bio-chamber measurements in inches,
and multiply all 3 dimensions---------------------------->12"x12"x12"=
to get a cubic inch number.----------------------------->1,728 cu in.
Take that number, and divide by 9.------------>1,728 divided by 9 =
This will give you the needed amount of SCRUBBIES.-----> 192
This will give you the # needed for a measured bio-area. As far as the # needed per gallon of tank/sump space, I remember seeing some recommendations somewhere, I'll see if I bookmarked it.
I don't remember exactly how many scubbies I added but I do remember I figured out I have about 562,000sq in of surface area with them. For 2 cats a pleco and a gar in my pond