I used epsom salt in my rainbow tank. My town has extremely soft water, and my rainbows were prone to columnaris infection if I let the hardness get too low. It worked and kept the fish healthy , but I didn't like that it only added magnesium, not calcium. Eventually I switched to "cichlid salt," (designed for rift lake cichlids) which did a better job. I understand seachem makes some good products that will raise hardness in a more balanced way. The cost might be prohibitive long-term.
Joey, I don't know your situation, but I always recommend changing your stocking to suit your tap water, rather than the other way around. You'll probably be happier in the long run.