If you do go ahead and media clean your tank, instead of filling with water, try this: take a flexible length of hose (shop vac?) and slide one end over an exhaust pipe (of a running vehicle, that is), and the other end over the fill neck of tank, I guess you could clamp each end also.
Saw a guy weld/ braze on a gas tank doing this. Apparently, CO2 keeps from explosion as there is no air (oxygen , neccessary for fire to burn) and fumes that that is what burns (just a fyi, liquid gasoline is non-flammable...it is the fumes that burn)