Derusting the inside of a tank can be a pain.
There are a few things you can do.
1. Cut the tank in half, sand blast or use electrolysis, then solder or weld back together.
2. electrolysis the inside, this still leaves the black steel inside though and needs wire brushing.
3. Use a tank sealer inside like this. http://www.kbs-coatings.com.au/gold-standard-tank-sealer-information-and-instructions

4. I have heard the product CLR Clear can help as well.
5. Put some rocks or steal nuts inside with some water, seal it up and shake it like a rattle.