Lots of farm diesel tanks are made out of steel, and have rust problems. I'd be sure and install a big diesel filter with any steel tank, and change it often. That said, I'd try naval jelly which is just a thick form of Phosphoric acid to dissolve the rust. Then rinse it real good. But since Diesel fuel does attract moisture, any steel tank will get some rust over time. That's why I suggest using a good diesel filter.
In one test I read in Successful Farming magazine, several thousand-gallon diesel tanks were cleaned out on several midwestern farms and they found buckets & buckets of rusty goo in the bottoms of them. But they all had good big filters and so the equipment wasn't damaged. But they did suggest not to draw diesel out of the farm tanks for 24-48 hours after refilling them to give the rusty sediment a chance to settle out to the bottom. That won't be a problem with your tank, but I just wanted to include the info.
Attention: TDR Forum Junkies 
