If your water is high in iron, buy pellets formulated for iron removal, like the type shown. Don't skimp by buying cheap rock salt. In the end, the money you save will be far offset by the cost of softener repairs or replacement.
At our house, we have a lot of iron in our well water. We know immediately when our water softener is out of salt because the water tastes horrible, and we instantly notice rust stains beneath every faucet or spout. Well, I've had to replace two water softeners over the course of the last 25 years. With the first two softeners, I made the mistake of using cheap rock salt and never cleaning the resin bed. Here are two important lessons I've learned:
1. If your water is high in iron, use high-quality, iron-removing salt pellets. Rock salt is far from pure, and the contaminants will eventually leave deposits on the bottom of the brine tank and clog injectors, valves and tubes.
2. Clean your resin bed once a year with a resin bed cleaner like Iron-Out. The resin bed beads have a negative electrostatic charge, which attracts positively charged particles like calcium and iron, making them stick to the beads. When the softener recharges, it flushes salty water over the beads to strip them of the mineral deposits and send them down the drain. Then it flushes clean water to get rid of the excess salt. But not all the iron gets rinsed off, and the beads become increasingly less effective. But that's not all. Because the beads don't bind all the contaminants (especially the iron), they're free to go on and clog the valves, tubing, O-rings and everything else. That's why the annual resin bed cleaning is important. For instructions on how to do this, see your owner's manual or read the label on Iron-Out or any other resin bed cleaning product.
The truth is, a water softener isn't designed to treat heavily iron-laden water, and eventually your resin bed will be ruined. But if that happens, you don't necessarily have to throw in the towel. It's possible to replace a resin bed. For bad iron problems, the preferable solution is to place an iron filter before the softener. Do that and your softener will last a very long time.
— Travis Larson, Senior Editor