Seriously, take the time and remove the tank and pressure wash it. You'll never get all that ??? out any other way, that I am aware of. You will feel a whole lot better about it, if you do a thorough job now. It's not an awful job; but the peace of mind, after you are done, is worth it. As far as the old fuel - just strain it (through an old t shirt for instance), and put it back in the tank. Leave the last bit of the fuel in the can/bucket, if you even THINK there is water in it.

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82 300D Turbo 124,000 mi.