Thus, for the 300-gal Central American Cichlid tank, you should get yourself a standard 10-inch water filter. What you linked to as the A-O Smith Clean Water System is intended for drinking water not fish tanks.
Consider these options:
API Tap Water Filter
Seachem Purfiltrum
Aquatic Life RO Buddie
Some factors that should however be considered while choosing include the flow rate of the filter, its life and the cost of replacing it. Also, it is pertinent to check that the filter is suitable to your Custom Aquariums 81 gal sump and the overflow drain.