What I've done before is, take some large nuts (like 1/2" or so) or large ball bearings, throw them in the tank with a gallon or two of diesel fuel or kerosine. Shake everything up and rinse and do it all over again. On the really sludged tanks, I get them boiled, you can't get that hard stuff out of the seams without a chemical. If the tank has a rust problem I would consider sealing the inside with a tank coating.
Good luck,
Scott
