The "Large Fuel Tank" takes a few shells to blow up, but I prefer to use the Oerlikon or Bofors AA gun on them to conserve ammo.
Some data that might help:
- Large Fuel Tanks have 100 hit points each.
- Your deck gun does 25 to 40 HP per hit (depending on how far you've upgraded it).
- The M2 Browning AA gun does 2 HP per hit.
- The Oerlikon AA guns do 7.5 HP per hit.
- The Bofors 40mm AA gun does 18 HP per hit.