I always add a bit of my native red clay to my dirt mix when I am setting up a new tank-about 5% more or less.
As my NPT's mature the rosette plants will sometimes start to look tired and I make native red clay balls and sun dry them so I can poke them into the soil around the rosette plants that look to need an iron boost.
This is one of the only additives I use in my NPT's.
Some clays are high in iron and other minerals, but some are not . If you can identify the clay in your back yard then you can find more information . I searched clay that is rich in iron and a couple sources popped up .