Most builders, Aquasport included, don't have a keel stringer under the fuel tank. They install tanks that contour to the hull bottom at the keel so as to maximize tankage. You should have outboard stringers, the stringers that form the port and starboard bulkheads surrounding your tank compartment, and then the inboard stringers fore and aft of the athwartships bulkheads that form your fore and aft compartrment bulkheads.
Typically, at the keel, the hull layup is about 1 1/4" thick. It really doesn't need a keel stringer.