You're looking for an acid that will prevent the undo the current corrosion of your irrigation tank, but not hurt the plants. Something like sulfuric acid should work well.
In addition, I would consider adding a sacrificial anode to your tank. You can use zinc rods of the type used for water heaters, as long as it comes in direct contact with the metal of the tank itself, and the water. It will also supply this vital micronutrient to your irrigation water, albeit in very small quantity.
Sorry, I just realized that the Iron is coming from the well and not the tank itself.
That is less fortunate and harder to solve. Ideally you would use a phosphate or carbonate to bind the dissolved iron in the water. This means potentially adding a supplemental stream that treats the incoming well water as it is pumped into the tank.
Alternatively you can also raise the pH of the irrigation water, but that will also result in micronutrient deficiency over time depending.
If you're considering those products specifically, there are a couple Saftey Data Sheets to consider.
http://www.sprinklerite.com/uploads/documents/GSX01018_SDS-BSNX104.2.pdfWas it Goof off or Rost off?