A bak-pak is not a filter, its a skimmer. It doesn't work the same way a Magnum 350 would.
A single CPR isn't enough skimmer for a 75gallon...I'd go with the Aqua C REmora Pro with the mag pump...or 2 CPR bak-paks.
With sufficient live rock (1-2lbs per gallon of tank) you don't need a mechanical filter, just a skimmer, and enough power heads to give at least 10 x per hour turnover (750 gph
- up to 1500 gph