how to clean nasty grime, lime scale crap off of a used tank ?

[Hopkins]

Moved to the current forum, as this is not an emergency.

I agree with all the vinegar and elbow grease comments, it works every time.

[AZDesertRat]

I use straight distilled white vinegar, its like $1.80 a gallon at Wal Mart.
For the sides I soak an old towel or rags in the vinegar, lay the tank on its side and spread the towel over the surface. The area stays wet and the calcium melts away quickly. Mixing the vinegar with water raises the pH so it is not as effective, use it straight.
Then after that, if its a glass tank I use a Scotchbrite pad to polish it up. If its acrylic you have to be more careful to not scratch the surface.