Fuel Tank Size

[Bushwacker]

In a boat that age, it's very possible that the original tank was replaced, even though SeaCraft never foamed in the tanks at the factory, which allows them to last MUCH longer, without wet foam holding saltwater against the tank. Replacement tank may have been smaller than original. Some 23 Sceptre/Tsunami's had 105 gallon tanks when new, although they may not have had the fishbox you mention. A second smaller tank might be a good option if you burn E-10 gas because it's best to have tank either completely full or empty with that stuff to minimize condensation, water build up and phase separation, especially if you have to mothball the boat in winter.