The short answer
Rust-defense salts carry an additive that helps keep iron off the resin; above about 5 ppm of iron, or with red water, an iron filter is the fix.
Rust-defense salts
Examples: Diamond Crystal Iron Fighter 40 lb Rust-Defense Softener Salt. They help with small amounts of dissolved iron the softener removes.
The limits
Nelsen notes most softeners are rated for about 2 ppm of ferrous iron; Penn State recommends softeners for iron only below about 5 mg/L. Above that, an iron filter goes first: iSpring WCFM400K Air-Injection Iron Filter.
Sizing with iron
Add about 4 gpg per ppm of iron to your hardness; size the softener with it.
Products mentioned
Questions readers ask
What salt is best for iron in water?
Rust-defense or iron-out salts help with small amounts of dissolved iron; they don't replace an iron filter.
Can I use regular salt with iron in my water?
Yes, at low iron; consider a rust-defense salt and periodic resin cleaning as the maker advises.