How much and what kind of K-media (bioballs) should I use for a 50 gallon aquarium?

[markfleener]

It's for a 50 gallon aquarium. I think the fluidized aerobic bacteria filter compartment for the K-media will be 7" x 12" x about 7" deep. Is that enough space for the K-media for this size of an aquarium? Should I use K3 or K2? How strong should the air pump be?