[BerkelUSA]

Draino with Lye and a gallon of hot water.. Wear gloves and NEVER seal the tank off.. (you will feel it generating heat), If it will take the rust-flakes off the inside of an old radiator it will do the same for a tank..

Then again there is the Molasses way.. (research that one as yes it works)

or reverse electrolysis from the inside?

WARNING: If your petcock on that tank is aluminum and you use muriatic expect it to be destroyed.. and the fumes can kill you..

Ever drop an aluminum carburetor in a coffe can of muriatic acid at the junkyard and leave it inside a car in the sun on a hot day?

When you return an hour later the interior may "look" good, but it crumbles when you touch it..

Rob