Ok, first off u need to get the rust clinging to the sides/bottom off. To do this, use a brush with a handle made of wire. You can then bend it into any position, and get too all areas of the tank.

Next, buy some caustic soda, fill tank 3/4 full of water, add the soda and swill it around for 10 mins. Then put it in the dishwasher. It doesn't make a mess at all - plates are dirtier - my mum even suggested it and she's a clean freak!

You have to physcially scrub it with wire brush though, the soda/dish washer will not remove the rust particles clinging to the sides. You don't need a lot of pressure either. I did mine last week, and it looks brand new