To clean the inside, I would just use vinegar. Lay the tank on its side and pour vinegar onto the glass. The lip should hold the vinegar in and basically give you a sheet of vinegar on the glass. Come back in an hour and turn the tank so another side is down and let that side soak. Keep going until all sides are clean. You can hustle things along by using a razor blade to clean the glass once the deposits have been softened up by the vinegar. Clean the tank out with water when you're done and you should be good to go.
As for the outside, unless there are deposits, water should clean it.
Since it's a used tank, also note whether you need to re-seal the plastic frame on the top of the tank so that water/salt won't creep out once it's set up.
I'm not sure I'd use any polish material. Vinegar and water are safe. It would be hard to determine whether you've removed all deposits from any polish.
Gary
