Did you "inherit" it -- or is it new? Was it empty before? Can you "see" into it so that you can deduce the condition of the tank?

How do you get the fuel from the tank to use it? (Do you manually pump-it into the maine fuel tank or "decant it"?)
drivi
Is this for use in a truck or car?

Simplistically, with a decent prefilter, you should be able to use the diesel in a non-fuel injected conventional engine for a while.

If you're talking about Houston, TX vs. the drier West Texas, then I think you might have a different story.

Are you just using this tank to extend your driving distance between fill-ups?