Well if it is a bolt then you could push it back into the tank because...
The defining distinction is in their intended purpose: Bolts are for the assembly of two unthreaded components, with the aid of a nut. Screws in contrast are used with components that have at least one containing its own thread. Many screws and bolts can be either, depending on how they are used.
Nevertheless - I have given my all - good luck with the bolt.
O Welles
October 2014