Draining the tank... Disconnect the fuel pressure line to the injectors and run it to a container. Connect the fuel pump to the battery and watch petrol go everywhere (@30 psi, or at least trying to get there)... Adding something to the fuel? Possibly it is 'just' water .. so some metho in the tank, shake and run. Usually water will settle to the bottom of the tank and that is where the fuel pick up should be for the fuel pump... There may be a paper fuel filter in there some where .. it will get blocked with water and it would be a good idea to have a spare available.