Start from what your fish actually need
Hardness is really two measurements: GH, the calcium and magnesium in the water, and KH, the carbonate buffering that keeps pH steady. Fish care about both, but not equally. Many tetras, rasboras and discus evolved in soft, mineral-poor water and often stay pale and reluctant to breed in hard tap water, while livebearers, rainbowfish and many African cichlids come from mineral-rich water and waste away in soft water, and snails and shrimp need dissolved minerals to build shell and carapace. Test your tap water first with a liquid GH and KH kit; plenty of keepers discover their supply is already close to what their fish want, and the safest adjustment is the one you never have to make.
Raising hardness the slow, steady way
The reliable way to harden water is to let minerals dissolve into it gradually. Crushed coral or aragonite in a filter bag, or a substrate of coral sand, leaches calcium and carbonate continuously, lifting GH, KH and pH together in a gentle, self-limiting way that is very hard to overdose. Limestone and dolomite rocks in the aquascape push in the same direction more slowly. Prepare change water in a bucket with a small bag of crushed coral resting in it so the new water arrives already close to tank conditions. Raise hardness over days, not hours: a fish moved between noticeably different mineral levels needs time for its body to adjust its internal balance, and a slow drift spares it that work.
Softening without shocking the tank
Softening means removing minerals, and only dilution truly does it. Blending tap water with reverse-osmosis water in a fixed ratio is the controllable method: half and half roughly halves the hardness, and pure RO water remineralized to a chosen recipe gives exact, repeatable softness for demanding fish. Peat in the filter or in the change-water bucket softens mildly and adds the tea tint many soft-water species seem to enjoy, though it is weaker and less predictable. Driftwood alone barely moves hardness despite its reputation. Avoid liquid pH adjusters sold as a shortcut: they fight the water's buffering, the pH rebounds within hours, and the fish ride a swing that is far worse than a stable value a little off their textbook ideal.
Match the change water, every single time
Whatever direction you are adjusting, the rule that protects the fish is that new water must match the tank before it goes in, in temperature and in hardness. Keepers who harden or soften only the tank itself subject their fish to a twice-weekly chemistry ride. Mix your change water in its bucket, test it, adjust the blend if it has drifted, and only then pour. If your tap water's hardness shifts with the seasons, as many municipal supplies do, the bucket test catches it before your fish have to. Consistency is the entire secret: a stable recipe repeated beats a perfect number reached once.
Keep the recipe where you will find it
Write down, for each tank, the target GH and KH, the fish those numbers serve, and the exact method that produces them: the blend ratio, the bag of crushed coral in the change bucket, the peat in the filter. Recording that recipe in the tank's KinsLe Pet Passport means a few months from now, or when someone else mixes change water while you are away, the water comes out the same as it did today, and your fish never notice a thing.