Hmm, last time I saw a bio ball in person I want to saw it was about an inch around, I highly doubt you'll fit anywhere near 400 in your filter.
If all you are after is more beneficial bacteria media in your filter, I would just get some lava rocks and use those.
Ceramic disks do the same thing, you can get those from the pet store usually, but they are going to cost more than the lava rocks but will be in a more uniform package.
Some people may disagree with me here, but a well used bag of carbon has been an excellent source of beneficial bacteria for me. I have a bag I've used for almost three years now. The carbon we use in the aquarium hobby has an extremely large surface area. Need more surface area? Go with activated carbon used in air purification, one gram of the stuff has about 2,100 square yards of surface area.