How To Clean Fish Tank Hard Water

How to clean hard water build up off a fish tank?
I have this 10 gallon tank, (not in use at the moment) that has very very bad hard water build up. What would be the best way to remove this build up, quickly and safely so I could put fish it there?
Use lime. After cleaning the tank. make sure to rinse it properly so that you get rid of all the lime before use.
-rds
Water change and rock cleaning. Reef Tank Cleaning