One way to add biomedia for cartridge style HOBs is by using ceramic rings and fishing line. Take a length of fishing line and tie it around a ceramic ring. Feed the line through a handful of rings and tie a knot around the last one. Leave some slack between the rings. Next, lower the string of rings into the HOB
in a serpentine fashion (side to side) in the space between the cartridge and overflow lip. You may have to experiment with the number of rings.
Having the rings on a fishing line makes it convenient for removal when doing maintenance on the filter or rinsing the cartridge.