One way as mutually suggested by the experts...
There are many chemicals that can help removing rust from the tank, you can use WD40 (1/2 Litre) OR Kerosene Oil (1-2 Litres) and put some chains, old nuts and bolts in the tank and shake it well. Then drain out the solution and make sure you take out nuts/bolts properly. Rinse the tank so that no rust partical remains inside. Hopefully it will eliminate most of the rust if the tank is not in a bad bad state from inside. If not satisfied, repeat the process till the WD-40 or Kerosene Oil comes out clear.