Evening Kevin
If it is just surface rust, internal of the tank, no oil or varnish contamination. Then I use warm water mixed with Molasses mixed at 4:1.
Use cheap animal feed Molasses from your local animal feed store it’s about £8 a gallon.
Mix it up with warm water, fill the tank and leave for about 48hrs, drain the tank into a container for future use on other rusty parts and flush the tank with clean water, it should come out back to bare clean metal but will rust again very quickly unless treated with an oily solution.
The chemical reaction of the molasses with (iron oxide), rust, makes the rust missable with water I.e the rust dissolves into the water. The longer you leave it the cleaner and it’s not toxic.
Note it only works with iron and steel, won’t work on Alloy, brass etc.
Not witch craft, honest.
Steve