The short answer
To soften aquarium water, dilute your hard tap water with mineral-free water — reverse osmosis (RO) or collected rainwater. Mixing, say, half tap and half RO roughly halves your hardness. You can also lower KH gently with peat or botanicals, but dilution is the most controllable method. Whatever you use, soften gradually over several water changes, because sudden swings in hardness stress fish more than hard water does.
Why you might soften water
Many prized fish — wild-type tetras, rasboras, bettas, apistogramma, discus — come from soft, acidic waters and colour up and breed best there. Softening lowers both GH (the minerals) and usually KH (the buffer). Just remember that lowering KH removes your pH cushion, so soft water needs a gentler hand and more careful testing. If your fish are hardy community species already thriving, you may not need to soften at all.
The reliable method: dilute with RO or rainwater
Blend mineral-free water into your tap water to hit your target GH. RO is the cleanest option; rainwater works if collected cleanly. Test the mix before it goes in, and remineralise very soft blends slightly so shrimp and plants still get trace minerals. See using RO water and is rainwater safe.
The gentle method: peat and botanicals
Peat, catappa (Indian almond) leaves and driftwood release tannins and mild acids that soften water and nudge pH down naturally. It’s slower and less precise, but wonderful for blackwater setups. It also tints the water amber — see removing tannins if you’d rather keep it clear.
Test everything with a liquid test kit, and see the reverse process in making water harder. More in the water testing hub.