how is it siezed is it a rust issue? i may have an idea you can try but you would have to clean the snott out of it after you try it... i use it on car locks that are siezed i set it in and over night it is free and clear. but i would clean it for awhile before use for the tank. its just a thought
http://www.nucalgon.com/products/coi..._foambrite.htm
i got it from a airconditioing guy.. again i could be on crack