If the tank is full, +1 on white vinegar and a single edged razor blade.
If the tank is empty any acidic type product - CLR comes to mind - just rinse well after.
In either case, a measure of elbow grease goes a long way.
Having just emptied and cleaned my 30 gal a few weeks ago I can confirm vinegar is awesome. I put some in a spray bottle and sprayed it on the calcium deposits - they started to fizz almost like when one add's baking soda to vinegar. Instead of using a razor blade I let the vinegar sit on the deposits for a few minutes and then just took a rough scouring pad and scrubbed the residue off.