Of course, no soap!
Ive never eard good thing about using chemicals, etc, so I guess, until someone corrects me, I'd BOIL (yes, boil!) some water, let it cool just slightly, and rinse the tank with it. Then, with SCALDING (but not boiling..) water, I'd leave the whole tank full with something over top to keep the heat in for a few hours, or til the water cools. Then rinse and scrub with a paper towel or brand new, unscneted sponge.
Then repeat a million times lol XD Over the course of a few days, it should be good.. Filling it up will also check for leaks.
If its really scuzzy, though...Maybe do it longer and harder.. ^^;;;
Great find btw :B